Cyclaholic- what about using a 26" wheel and some "road brakes" if that can be found? You can buy some special brakes that can be used for two diff sizes of wheels, but hallf the fun is to make it low cost.. Maybe make some sort of extension to the brake arms? Then ofcourse there is the hub breake posibility.
I am lucky, there is a unlimited access to dumpster bikes in the area. I pull them out and give them to the refugees in the area, and I find all I need for my projects. Also wheelchairvheels for trailers.
Do you think your rear triangel would be stronger if you put a seattube innside again? Maybe the top of that seat tube could help you find a way to connect to the main frame, especially if you need to go "weld free".