Both the MT-32 and SC-55 have their own volume control, yes. I believe there is already some precedent in MAME for machines interacting with each other.
Some people have actually talked about having a virtual desktop where you can plug in devices, power things on, &c. I think that's a little much though.
I think this sort of thing, along with some of my work I hope to integrate in the future, could be easily handled with a tool that makes signal connections between various components with a GUI (kind of like a CAD program). It would look like a block diagram, where the blocks are various devices, and the connections must match by signal type. This would be a front end that generates some sort of declarative description of the system you want (e.g. XML). I think this kind of system would be intuitive for users and could possibly even be used for emulation development (e.g. a machine would consist of CPU blocks, memory blocks, ROM blocks, video signal and audio signal outputs, &c.).
But for the short term, I'm sure slot devices or something will work fine for these devices, and if there is something inadequate, it could easily be expanded.