Although I can't seem to find a microSD to RS-MMC adapter readily available, after lots of searching, I've learned the following things:

1. All SD cards (including microSD and miniSD) are required to be backwards compatible to the original MMC protocol.

2. The dimensions (Width x Height x Thickness) of an RS-MMC/MMCMobile card are 24mm x 18mm x 1.4mm.

3. The dimensions of a microSD card are 11mm x 15mm x 1mm.

4. Noting #1-3 above, it is feasible that a microSD to RS-MMC adapter device could be made. MiniSD is too long and thick to be used. (20mm x 21.5mm x 1.4mm)

5. Any card above 2gb would require kernel modifications to accommodate the increase in size. Similar modifications have been performed on other embedded platforms that were limited to using MMC mode with SD cards (OpenWRT-compatible devices for example).

Anyhow, heres hoping my findings can be the start of getting a 4+gb flash card for the 770.