This comes up once and a while. The real solution is on the way and that is a 4 channel board I have in the works that uses the PGA2310 chip and some nice buffers. This board will be plenty good for rear channels, subs, etc. and will be part of the TAP system - but much lower cost than 4 channels of TVC. I feel that TX102's are not needed for rears, etc. The nice thing about this board is that it will allow for full trimming of the surround channels to match the tx102's on the main channels.
I had seen the DACT setup and it is one of the things I had in mind when I made the DACT attenuator remote. What you describe could be built but it would still not be cheap and it would not allow for channel to channel trimming - which would likely be an issue unless amps and speakers are all matched - or unless the amps have level controls to allow channel trimming.
Please describe your whole system (or intended system) and then we'll see what the best plan would be.