Do VPLS and VPWS services transfer STP frames? How about other L2 control frames? Can two geographically dispersed sites have one STP region over any L2VPN?
Normally yes, VPLS and VPWS should be transparent to all L2 frames. BPDUs might not be supported by all providers, so you'd want to check the SLA.
Alternatively, you should consider routing your traffic instead of bridging it. Routing removes the need to block redundant links and makes them productive.