The PhaseRunner's ebraking input voltage swing is backwards, so it goes from 0.8V for ebraking to start to 0.0V for full ebraking, so the scheme used will have to account for that. An opamp can be used to invert the signal from a thumb throttle and scale it appropriately. A CMOS analog switch could be used to select which signal to apply to the PR. A Comparator could be used to determine the level and switch between the two inputs at say 0.9V. The problem with analog circuitry is in the details. Getting proper power (possibly bipolar), filtering, shielding, etc. The CMOS switch is susceptible to static discharge. With the various bias resistors, bypass caps and voltage regulation it quickly gets to a number of components. We used to do things this way but not these days.
One way to handle this is to use a small micro that has a DAC output, or a separate DAC chip that the micro drives. Use two analog inputs, one for throttle and one for ebrake. Then write a few lines of software to read both ADCs and map the output DAC appropriately. I would probably use a small Arduino for this, but most anything could do the job. A Raspberry Pi would be total overkill, but whatever is easiest for you. The nice thing here is you could determine the priority so a signal from the brake input overrides the throttle. I built something like this but have never put it on a bike. One more thing to package / mount / power / cable / connectorize / etc.
I've been riding the Borg lately, and it has the variable ebrake setup since there's insufficient space for a rear brake. It is a little un-nerving the way it is working. I have the "drag" or "slip regen" feature enabled and set to a modest level of regen, so releasing the throttle causes mild regen, similar to engine braking. So here's what happens when you come to a stop. First you release the throttle and get mild regen. Then you pull the rear brake lever, and as soon as the switch activates the regen goes to zero, it feels like a mild acceleration, exactly opposite of what you want (rather like pulling the clutch on a motorcycle). Then you pull the brake lever farther (with essentially zero resistance, it just moves) and regen goes from zero to very strong braking, but with a little delay. At very low speed the regen reduces and drops out (like 2 mph). Then the front brake (dual disc) is used to bring the bike to a stop.
When first pulling the ebrake the slight acceleration is unnerving. I wish it would transition smoothly from drag to variable regen. This could be fixed with a circuit that sets the minimum ebrake to be equal to the drag. But controller settings don't seem to cover it.
In general the ebrake doesn't feel quite like a brake. The lever moves a long way with little resistance, there is delay, and the braking varies inversely with speed. It is wonderful, but it is not what your brain expects for a brake. You have to adjust, and with the delay it is easy to think it is not working for a moment.
I tried putting the ebrake on another throttle. I did not like that at all. It was yet another thing to think about, slowing down the process and requiring new motions that are not automatic. The brake lever, even with it's flaws, is more natural and automatic. Years of motorcycle riding has burned the pathways into automatic reflexes.
The RadCity's front spokes are rattling due to being loose. It is like they built the wheel but forgot to tension it. The rear is better. I think I'm going to try slightly tightening an even amount on every spoke. Running it the way it is may be bad for the wheel. I haven't tensioned wheels so I'm going to go slow and then get it tensioned properly on Sunday. With these thick 12 gauge spokes it is easy to go from loose to pulling out the nipples and warping rims, so going in baby steps is required.
We use Topeak MTX compatible trunks, and they have a dovetail mounting system. The rack on the RadCity has a lot of features but not a Topeak track. Topeak doesn't seem to make a track that can be bolted to a rack, strangely enough. I searched and found a design on Thingiverse for a compatible 3D printed track. I also started a design of my own. I may put one or the other on the RadCity. That could be very handy.