Performance Question

[Rob Brackett]

How hard would it be to modify the MySQL code, and what sort of
performance hits would I take, if I was to try to run a hundred thousand
MySQL DBs on the same server?  Obviously, some tweaks would be necessary
to be able to break up the directory structure of /var/lib/mysql, and I
may be souding uninformed, but would there be much to do beyond that?

Kind of a pie in the sky question, I know.

-Rob-