If you want to get the OSPF as E2 route from one vpn site to another, the answer is yes , you can ask the SP to play with instance id.
I have posted the same in blog:-
http://shivlu.blogspot.com/2008/11/why-ospf-vpnv4-routes-look-as-external_07.html
From another vpn site you can change the external route to E1 during redistribution.
regards
shivlu jain