My thought is this: can you use one of Parallax's cheap ($2.79) controllers to build a ROM cartridge replacement that would store programs in cheap I2C serial EEPROMs?
Here's a TinyURL for the product page: http://tinyurl.com/5ug7ym
Not only could you make cheap multicarts, you could also build I2C interface projects for temperature detection and logging, A/D conversion, and other cool stuff. You could even (I think) create a fast RS-232 interface without using a MAX chip.
Could you even emulate a Commodore REU DMA controller with this?
Okay, you hardware guys (who are all way smarter and better looking than me), discuss...