Yes, it is certainly possible to format a single memory card with two separate file systems. They don't even have to both be the same type. In fact, that's the premise of the "extended root file system" hack that was popular on the 770.

The problem, as jpj stated, is that the Nokia 770 (out of the box) won't automatically mount both of the file systems. It will only mount the first one, and only if it is a FAT file system. It is possible (with some as-root hacking) to set up the 770 so that it automatically mounts the second file system; it just won't show up in any of the UI's, which is fine for Maemo Mapper's use (you can still use that second partition to store a Map Cache).

One strategy that you can use is to dedicate that second partition just for maps, sized exactly to the amount of map storage that you want. Then, you can write a maemo mapper startup script that will first mount the maps partition, then start maemo-mapper, and afterward unmount the partition.

Has anyone considered using jffs2 (the format that the 770/n800 uses for the root file system) for a map partition? I'm not entirely sure, so I can't confirm this, but I think jffs2 would not suffer at all from the large-block problem, since its blocks (or "nodes") are variable size.

It's not as convenience as FAT32 (not accessible or even formattable without MTD), but maybe it's worth investigating, if someone has the time and mkfs.jffs2 handy. If you do investigate, I would advise against using compression, since the PNG and JPEG images are already highly compressed.