MySQL databases on their own separate Logical Volume
This may sound twisted but here it goes....

DON'T USE CREATE DATABASE

If you login as root and you do the following:

cd /var/lib/mysql
mkdir db1
chown -R mysql:mysql db1


you just created the database. The INFORMATION_SCHEMA database will dynamically register db1 in the INFORMATION_SCHEMA.SCHEMATA table as a database.

Since you want to create a volume for it, you can do this

cd /var/lib/mysql
mkdir db1
mount /dev/sdb db1
chown -R mysql:mysql db1


GIVE IT A TRY !!!

Bad News: Backing up /var/lib/mysql will not scoop up mount volumes. You will have to resort to doing mysqldumps. That should not be an issue if you have 5GB mounts.