I think a SUSPENSION SEAT would be more practical, and allow the traditional use of that space for "stuff".
As an e-Fatbike hardtail owner, I can say a front suspension with a quality suspension seatpost and hardtail solves a lot of problems, without creating many new ones. Front Suspension + Hardtail + Suspension Seat is a formula that works, and could be translated to trikes & tadpoles.
I've been following this thread & product with interest, which has partly helped me decide to go with a trike (no purchase yet). Mainly, I got tired of falling over while stopped, stopping, turning around, and also my bike falling over while I was off of it. Got tired of tall standover, stepping over my top bar due to storage, being up high, and ergonomics that put stress on my hands/wrists and butt/perineum, even after extensive mitigations.
This is as good a place as any to share my ideas for a KMX or tadpole SUSPENSION SEAT. If someone goes into business from one of these ideas, I'd ask that at least, you'd give me one for free. I've put a lot of thought into this. I'm creative, but not that handy.
-VERTICAL SUSPENSION SEAT MOUNT
The KMX steel frame line uses a bracket to bolt a tubular seat frame directly to the rectangular frame tubing. You could have a device in the middle (it bolts to the frame, and seat frame bolts to it) that has a shock absorber system which allows for vertical travel. Similar to the optional Benz Sprinter van suspension seat (which are fantastic, btw). The Sprinter suspension seats also have a knob you can twist to make the suspension softer or firmer, depending on weight and preference. This would raise the seat height a little.
-REAR SHOCK SEAT
This would be a seat mount where the front of the seat is attached to a horizontal BAR, and the front swivels or tilts from that bar. The rear of the seat would be mounted to SPRINGS or SHOCKS (say, one on the left rear and one on the right rear; or maybe just one central shock absorber behind the seat). So, the seat would tilt back & down, swiveling from the nose of the seat. Since the shock absorbers would be in the back of the seat, it would primarily absorb jolts from the REAR wheel--and it would make a good complement to adam333's front suspension system! This concept is so simple, I could almost even do it.
This concept could also work on the KMX Koyote (folding, aluminum-frame) model, which uses more traditional tubular bar and mesh design for the seat (Koyote design seems to be "inspired by" the Catrike Villager--which isn't a bad thing, in my opinion). I'm strongly konsidering the Koyote because:
1) it's the tadpole which seems to have the most opportunity for easily making a flat "floor" and underseat storage (such as: wire grates like oven or refrigerator racks, or simply wood), and
2) Koyote is probably compatible with adam333's front suspension system [ADAM COULD YOU CONFIRM?]. I actually dis-like that it's made of aluminum, and the folding mechanism worries me (for strength), though it could be convenient. The design, while not as robust as the steel non-folders, still seems stronger than a Catrike, and has more potential for storage than either. (And neither the steel KMX nor Catrike Villager fold, FWIW.) My motor ideas would be like 72V with a Mac 10T, or maybe (less likely) one of the popular "35mm stator" DD (à la "Leaf motor 1500W", etc). Not necessarily crazy hard riding, but faster cruising.
So with the Koyote, it would be relatively easy to mount a swivel bar across the top-front of the seat tubes, and mount a suspension system in the back to the frame, or to the rear seat tubes. You would need a 3rd-party seat that sat inside their seat frame.
-BUNGEE/SHOCK CORD SEAT
After lots of thinking, I came to the conclusion that with the kmx Koyote, the easiest type of suspension seat that I could do myself, would be using elastic rope.
1) SUPPORTING THE MESH SEAT WITH BUNGEE CORD
-For Koyote: you could criss-cross (or some other pattern) shock-cord material between the seat-bottom tubes, and even the seatback, and place the mesh back on top of it. I've heard the Koyote mesh fasteners loosen after awhile, so that could also help as a support backup.
-For KMX steel frames: that tube seat frame seems pretty small, but maybe it's possible to suspend elastic between the seat frame, and somehow suspend a seating surface? I don't have one. Or, maybe a completely new/3rd-party tubular seat frame could be fitted, using this concept. Somehow, I'm sure it could be done, and much less elaborately than a rear suspension. Even a completely new, commercial upgrade seat could be easily made to mount to KMX's steel frame. And a totally new seat would still probably be less expensive, and certainly easier to implement, than rear suspension.
2) SUSPEND A SEAT FROM WITHIN THE FRAME
-For Koyote: use shock cord/bungee material to suspend a camp chair from between the Koyote seat tubes (in my mind, the Koyote is easier to come up with ideas--far more attachment points).
Basically, we are trying to adapt the concept of a bike "suspension seatpost" to a trike/tadpole design. Also, some of the above ideas, commercially or just DIY, could work on other trikes.
I have a $20 Schwinn cruiser spring seat (large springs that actually do something), and originally tried to come up with ideas of how I could cannibalize that seat--or maybe 2 of them--to a KMX design. After thinking about it a lot, I eventually had the suspension shock cord/bungee rope revelation.
Even before that, I first wondered how you could use 1 or 2 cheap suspension seatposts, but I realized suspension seatposts probably require too much height. Though, if you did a tilt-back/front-swivel design, you could use them as replacement "seatback support tubes"(?).
ADDENDUM: REAR SUSPENSION ON THE KOYOTE?
Adam's latest contraption got me thinking!
The Koyote ALREADY has a "cut" frame, since it has a folding design. No cutting! No "point of no return"! :) And while aluminum, that "cut frame" is already reinforced at that junction, and designed to be separated.
Might a bolt-on, adam333-style rear suspension be more easily adapted by novices to the Koyote's folding frame? Or does the aluminum frame automatically disqualify it? Adam, what do you think?
I hope this inspires, as I've certainly been inspired.
Adam's and EvolutionGTS/Electric Racing Technologies' designs have certainly been inspiring. I found Adam's designs through EvolutionGTS's "10KW Death Trike".