Innodb
Doesn't innodb's way of putting all the innodb databases in a single file, controled by the mysql config file prevent you from
a) control quotas through filesystem quotas, needing additional, in-mysql quotas
b) if you found a way to get seperate innodb databases to clients, wouldn't you need extra config work just to integrate all that?
You might fare better integrating bdb into mysql, as it at least has seperate files per customer(although I've worked with innodb and was satisfied, it was on an internal use machine)