The problem I have with these kinds of ideas is that you'll end up with a 'modern' computer with a very slow old computer attached to it
The main appeal of Classic hardware to most people is in running the original 68k games/demos/apps and a modern expansion doesn't fit into this appeal.
I'd personally draw the line at a really fast accelerator with an on-board HD controller but that's just my personal opinion. I think as soon as you replace the original chipset (eg AGA) then it becomes pointless (see above).
