The transfer cable part of it is pretty straight-forward, it's a MAX202 chip w/ 5 capacitors (one of them is a bypass cap) hooked up to Tx, Rx, and GND on the PC's serial port. On the S/NES side it goes to data (receive), strobe (send), +5V, and GND.

Then the NES cart needs one EPROM and 2 RAMs. This part is maybe a bit more complicated, at least it could be if you want to load a decent-sized program. OTOH, a board that already has CHR-RAM and PRG-RAM (SNROM, for example) would work fine for loading/running 8kB programs by only adding an EPROM without other modifications.

I'll look into making a development cart to work with it, like tepples suggested. I could supply the code for it if you want to build your own. It's based on this:
http://www.6502.org/source/io/xmodem/xmodem.htm

It would make for a nice minimal dev system. Any larger than that, and an EPROM emulator or CopyNES-style system becomes a better choice.

The thing that I really like about this cable though, is that you could use it within the program itself (even optionally for little things like saving high scores without a battery). And I've found a source for NES and SNES connectors, so I don't even have to ruin any controllers to make however many of these are needed.

It's like an accessory for what the Nintendo's "Programmable Series" should've been. :)