This is turning out to have a few more steps than I originally thought, which is pretty typical. 0-] The problem is in the interaction between the unpack process and the install process. The unpack is doing the actual work, so it needs to be able to switch discs and mount the new archives. The install process is the one that handles messages and interaction with the user. These two processes weren't really built for two-way communication, so they're not setup to handle this.
It's not a hard problem to solve, just one I need to think about the best way to do it. I've got a handle on it, so I'm still hoping to get you something this week. The good news is, once I'm done, your stuff should just work with the setup you have now. The installer will notice that the next file it's trying to install is missing and automatically prompt for another disc, and once inserted, it will mount that disc and continue on.