The issue with these things is that you quickly reach a point where the bicycle is almost entirely impractical for pedal only use due to the very high weight. That may be OK, but you do lose one area that could make an e-bike "unkillable" - for instance if a motor fails.
A Lohmann engine combined with a 1/2 gallon fuel tank will weigh all of about 15 lbs when loaded with fuel.
I found my coroplast-shelled velomobile trike that weighed 91 lbs plus loaded with 30+ lbs of gear to be very pedalable with the motor turned off. Including the cogging torque losses of the Leafbike 1500 4T motor in a 26" drive wheel that was in it, rolling averages of 15+ mph were not a problem for me on a disabled motor without the computer on to cancel the cogging torque losses. 9-10 mph rolling averages were still the case if there were a lot of stop signs/red lights at the bottom of hills. This also meant cruising 20+ mph on flat ground. At one time I had DOT rims and Mitas MC2 tires rated for 62 mph on the front wheels, and its rolling resistance at a steady speed didn't noticably change from the Schwalbe Marathon Plus 406 that used to be on it; it just took a little longer to accelerate. I eventually had 16" DOT rims and Mitas MC2 on all 3 wheels with a Leafbike 1500W 3T in the rear wheel and it didn't slow me down much, if at all. But I'm also fit and had a low enough granny gear to climb up a 20% gradient without my cadence dropping below 60 rpm. And I am light, around 140 lbs.
That's
very usable as a "bicycle" on a disabled motor.
My 90 lb quadricycle, "Satanic Panic" did run dead on the Katy Trail 2 weeks ago, and I was still able to lug that thing home at 10 mph for another 20+ miles, already tired after having ridden for over 90 miles using the motors to assist. This is with the cogging torque losses of two Grin All-Axle motors slowing it down, loaded with 30 lbs of stuff in the crate, and no aerodynamic shell. I did have Schwalbe Marathon Plus tires on it.
Also quite usable.
I think a 150 lb bike would be still very pedalable. This weight might describe a 4-wheel velomobile with electric mid drive motor in the back, hub motors in each front wheel, small ICE or diesel, pedal drive, solar panels, roll cage structure, hydraulic brakes, DOT rims, solar car tires rated for automobile speeds, and an additional 30 lbs of camping gear/supplies on board. The aerodynamics could be refined well enough that someone could maintain a 15+ mph rolling average and cruise 20+ mph on the flat with everything turned off if they are as physically fit as me. I'm not an olympic ahtlete or even a professional cyclist, for whatever it's worth, but I'm definitely more fit than your average Joe. Even then, use 10-30W to cancel out the cogging torque losses of the motors, and when coupled with a slippery body shell, I think even at 300 lbs laden(bike + me + gear) I could probably hold 20+ mph rolling averages in such a vehicle and cruise 25 mph on the flat. On dead battery/electronics, maybe the rolling averages will be 10-15 mph with 20 mph cruising speeds on the flat. Just a guess because I don't have this hypothetical vehicle completed yet. The "bicycle" element of its operation will still be well in tact in that case, whether I'm cancelling out the cogging torque losses with a small amount of power, or operating it dead.
The typical moped is around 200 lbs and has very lossy tires, for reference. I find those borderline pedalable without the ICE in use, and at like 5-7 mph on flat ground, to give some comparison.
Tires, rider position, and aerodynamic drag can make all the difference. And the rider will have to use their gears and learn to be patient going up hills if they're riding unpowered, and SPIN in order to save their knees.
Another bonus of a pedal powered drive system is that you can pedal it up a mountain, and then use regen to refill your battery pack if all else fails. I took advantage of the rolling hills in my city once in my 3-wheeled electric velomobile after my charger broke and I had to wait on a new one. I pedaled over 20 miles to a bike shop to get parts, using regen on the downhills, but operating with no power feeding the motor. During that ride, I recovered about 5 miles of range at 30 mph, which I needed in order to make it work on time at a later day. That's also handy.
The main purpose of such a vehicle is still to be riding high speeds using an electric motor. My quad typically cruises 30-35 mph, and often does 55 mph on the highway when I need it to. But it's nice to never be stranded because the battery ran dead, or if the electronics ever fail. I can still make it home, it will just take a while, and I might have to get on the sidewalk in some places. If we ever suffer a severe Cuba-style fuel crisis where nothing is available, the sorts of vehicles I've desribed would be invaluable.
