running postgresql database from loopback filesystem?

I have virtual hosted users on a server and some of them have a
postgresql database. I'm concerned about the outside possibility that a
user could create an infinite loop and fill up the partition on which
everyone's database resides.

Anyway, it occured to me that I could create a special file for each
database user of limited size, and make a filesystem on top of that
file. The file could be owned by the user. Then, each user could have a
seperate database and that database would be at the location of the
mountpoint of that filesystem.

It seems like a strange thing to do (an understatement, to be sure), but
is there anything wrong with it? Are there performance considerations
when using loopback devices like that? Are there dangers of corruption?
How does journaling (ext3, reiser, etc) work in that regard?

Regards,
Jeff Davis