I have not looked at the service manual for the brakes, will need to do that. But, I doubt that the brake distribution is controlled electronically in normal braking when regen might engage (e.g., when no ABS or VSC engages). It is most likely a static hydraulic valve like it was on the Camry, which simpy proportions the pressure from the single master cylinder. Brake by wire is there, yes, but I think it simy replaces the mechanical link to the master cylinder, and doesn't control the rear brakes any differently.
The rear drum brakes on my Insight do engage even with fairly light braking, so there will be no under use issues if converted to disk.
The drum brakes on the Insight behave nicely in most cases, except slowing down more intensively on steep downhills from highway speeds, especially with the car loaded. Even normal highway braking from high speeds might trigger the rear brake fade or whatever is the cause for the "rumble" that develops. Try it, I'd be surprised if you don't experience it.
Anyway, I'm more interested in sourcing a set than discussing theoretically if it might work or not. If someone has info whether the swap is a direct bolt-on (like on the Camry), I'd be interested to hear. All I had to do there was swap in some factory parts from the donor car, no other modifications of any kind were needed. If it's as easy and cheap on the Insight, I might just try it and if it turns out there are any hard to resolve issues, undo it ...
I'll do some digging in the service manual to see if I can learn anything insightful from there