which presumably will also intercept calls from within Class::DBI, any potential problems (such as transactions/rollbacks)?

Same caveats apply as before.

Aslo, can one use Apache::DBI outside of the Apache/mod_perl environment?

Looks like no.

can I somehow use Apache::DBI or something similar to provide connection pooling/reuse?

Yes, you could UTSL and do exactly what Apache::DBI does (just substitute the mod_perl bits for some POE bits), but since Class::DBI uses Ima::DBI, you don't need to do anything since Ima::DBI already does caching.

update: a casual google for Apache::DBI and Ima::DBI reveals an important caveat and patch.