When you configure your profile import  connections in the shared
services provider, you can define custom connections. These can be set
with a "search base" pointing to a "directory node" (which presumably
could be an OU). The search  can be to one level or the subtree.

If you need multiple OUs, you could define multiple connections. You
can also apply an LDAP filter to a connection (see
msdn.microsoft.com/.../aa746475.aspx?ppud=4 for
examples). So you can exclude (for example) machine or disabled
accounts.

Couldn't you alternately modify your people  search results screen?
You could create a custom Scope that only has those particular people
in it. Then, modify the people results page so that the (people
results web part) so that it only returns that one scope (in the misc
section of the web part properties).