I happened across a linked article mentioned this week at the
OpenBSD Journal, about using virtual routing domains -- and the article suggested the possibility of using them to connect with multiple ISPs, though it did not show a PF ruleset that might be applied in the solution.
This might be a way to circumvent your apparent pppoe restriction.
The article page provides a contact link for the author, as well as a comments section.
http://www.packetmischief.ca/2011/09...routing-table/