On the whole, it's not bad. But given that your plan is to directly interface the Rackbrute 3U to these other sources, I'm kinda of the opinion that creating something less audio-oriented might be a better idea. There's a lot of modulation and ancillary modules that could populate the case and having that as a 'nexus' for the other patchables seems sensible, especially since this would be part of a rig where there's already three patchable synths specced out. Plus, if you used the Keybrute direct into the Rackbrute cab, that could serve to branch and reprocess the control signals coming from that. To start with, go and have a look at the Expert Sleepers FH-1. That module is a USB host-to-CV/gate...which would allow you to directly interface the Keybrute into the modular setup, and from THAT you could process and branch (with polyphony, in theory!) to all of the other synths.
Remember: just because it uses synthesizer modules doesn't necessarily mean that it has to make sound itself!