I wonder modifying a small MicroSD to SD adapter would work. Surfing the net I come across a cheap adapter like this:



One side of the adapter is the USB insert, and the other side is SD. Without the USB portion the length is around 21mm, slightly longer than the 18mm RS-MMC standard. Someone reported that it's a bit thicker than the normal SD card, and if you crack open the case it looks like this:



So if the case is removed, usb part is cut off, and glue a MicroSD onto the PCB, it might be thin enough and small enough to insert into 770's RS-MMC socket.

PS I don't want to spam the forum, so if you want to know where to buy it, google "Worlds Smallest 2-in-1 Trio TransFlash USB Card Reader".