How to require an MB subclass in a CPAN friendly way?

Here's what I've got so far:

	http://search.cpan.org/src/CRAKRJACK/Schema-RDBMS-AUS-0.01/Build.PL

I'm forcing the "install" action to fail without the Module::Build subclass
in DBIx::Migration::Directories (which I've noticed lately has a few
similarities with App::Build), because without this subclass, the install
will be useless to a user.

The problem is that if somebody doesn't have DBIx::Migration::Directories
installed already, they'll have to go through CPAN twice, since CPAN won't
re-run Build.PL after downloading the dependancies.

I see this being a problem in the mod_perl world as well, with the
Apache::TestMB Module::Build subclass.

I know this has been discussed a little bit before, if there's a mailing
list archive message with a definate solution or something just point me
there and I'll shut up. :) 

Is there a solution that will make sure that a package using a MB subclass
from another package installs cleanly out of CPAN, whether or not that
subclass is already installed on a system?

	Thanks,
		Tyler