I think your charging idea is what some company is doing in Austin, Texas. They do this for the Prius. I thought I read they change out the original batteries, modify the ECU and fix up a plug in charger. It's costly though but some are getting over 100 mpg with this change.

I had thought about a charger that could take the batteries to about 80% charge. The batteries are designed for a quick charge. A charger that would bring the batteries up with a 12 hours charge should work. I would feel more comfortable monitoring the HV batteries for a month or more to read the max, minimum dc volts.

I started messing with nickel hydrid batteries when they first came out about 15 years ago. No memory and they had lots more ma than the nicads. I used them in my pylon model plane. I remember when I stopped flying the new quick charge hydrids were available. The batteries in our cars are hydrids specially designed just for hybrid use.

In the EV Mode the batteries seem to go farther if you drive 25 miles per hour (using cruise) instead of 40 miles per hour. Not many places you can maintain that slow speed for any length of time without getting run over.