You really wouldn't want any sort of expansion that is in constant use sitting on the bus that the expansion port uses. The bus itself is flaky as hell, and very picky about how things go across it. Not to mention how god-awful slow it is in comparison to the CPU-HOLLY bus (HOLLY is the ASIC that contains the PVR and a bunch of other logic... in case you aren't familiar with it). Oh yeah, and anything that does go out on that bus has to go through HOLLY as well.
Basically, unless you put a whole separate system on the other side of the expansion port, its pretty much useless. If you're going to do that anyway, why would you want to saddle the Dreamcast to the side of it? Sure, its an interesting EE project, but it serves pretty much no purpose, unfortunately. At that point, you wouldn't even really be able to call the monstrosity a Dreamcast anymore.