I think a project like this would be very interesting and worthwhile for many reasons - not the least of which to enable individuals or groups to build specialized solutions of "one" that the industry's main vendors can't go after due to lack of market size etc.
I've been working on a personal camera project that I'll be sharing soon and from that experience I believe that a 135 size "medium format" type back would actually find quite a sizeable market. I actually think that 135 chip size backs will be the new "medium format" for technical view cameras of the future (I am not saying that larger chips won't have the known advantages, but also some disadvantages).
Based on the learnings from my project, I am confident to say that 95% of current manufacturers got it wrong when it comes to technical cameras for next generation digital.
I'll be sharing my project here soon which will explain a lot of the above.
For my project I looked into a us vendor called Illunis ( http://www.illunis.com/
), they - for instance - create a 70mp 135 chip size "camera" that is pretty much exactly what you are looking for as a sensor "module". It interfaces via usb3 and standardized protocols with whatever processing unit (computer) you want to bring to the table. I've talked to their experts and they said the module would initially go for around $10k but would go down with time and volume. They also offer shutter modules etc as well as larger size sensor "cameras".
If I'd be spearheading this a project, this is what I'de be starting with for a good jumpstart. If you get a big enough community behind it - who knows Illunis might be supporting it.
I think the possibilities would be almost endless and many people will be surprised what is possible when commercial and marketing constraints are removed (i.e. closed vs open protocols, open firmwares, total mechanical freedom...)
Will it be hard, frustrating and sometimes seem impossible - yes for sure. Will it a lot of fun, yes I am sure as well. Can it work? Yes, with the right people at the table.
I hope this takes off and I am willing to help where I can.