I've been using ModRana for the past year, since Mappero (last I looked) was no longer being maintained. It supports multiple download sources, and can store things in sqlite database format, which is both compressed, and gets around losing chunks of space on every tile based on FS granularity.

Also: Google has become quite a pain when it comes to tile cacheing. If it detects you're doing it (too many too quickly) it cuts you off. Silly, since it means you're probably going to lessen the load on their servers over time, but then they want to control when/where/if tiles update. :P

Thanks for the refresher/tutorial on creating loopback file systems though. I'm sure several people have uses for such things!