I would like to encrypt traffic between two data centres. Communication between the sites is provided as a standard provider bridge (s-vlan/802.1ad), so that our local vlan tags (c-vlan/802.1q) are preserved on the trunk. The communication traverse several layer 2 hops in the provider network.

Border switches on both sides are Catalyst 3750-X with the MACSec service module, but I assume MACSec is out of the question, as I don't see any way to ensure L2 equality between the switches over a trunk, although it may be possible over a provider bridge. MPLS (using EoMPLS) would certainly allow this option, but is not available in this case.

Either way, equipment can always be replaced to accommodate technology and topology choices.

How do I go about finding viable technology options that can provide layer 2 point-to-point encryption over ethernet carrier networks?