Architecturally, it makes no sense to do a/d/a conversion anywhere but the backplane. This is the only place where it is possible for the modules to "talk" to each other without crosspatching. Placing the central "brain" as you put it in another module is a pointless waste of space, because unmodified the current system will not support either option. No module can get signal from any other module without external interconnects. If I'm going to design (and ask you to buy) a custom backplane I might as well put a/d/a conversion on it and not waste any module slots.
Let's clarify your proposal a bit. Is this what you want?
http://www.burlaudio.com/products/b80-mothership
If not, why?
I proposed this because I largely share your concerns re modularity of application and price. From what I've heard so far out of the senior people here these are entry-level/mid-level concerns. Things like the aforementioned JoeCo box do one thing but they do it very, very well. Simplicity and reliability is favored over flexibility and modularity.
That aside, let me make 2 things absolutely clear. A) If 10 more people show up here asking for the exact same thing you want, I don't care how far it deflects from my original target. I'll throw up a kickstarter or start design on spec, but as I already said I'm here to be helpful and not to stroke my own ego. B) I am more entry-level than you may think. I can only give you progress reports and the roughest of time estimates. There is no way i can guarantee to meet any deadlines whatsoever, so you'll have to be pretty patient.
Thanks again for your comments. I'm always interested to hear what people are looking for.
EDIT
Re paying for firmware updates, a non-issue. The IC does what it does.