Is this really an issue? So long as they're not advertising the space I see no issue with routing traffic through a 10. network as transit. If you have no reason to reach their router directly (and after Cisco's last exploit, I'd think no one would want anyone to reach their router directly :-) ), what's the harm done?
RFC1918 merely states that it shouldn't be routed on the global internet, not that it can't be used for transit space.
RFC1918: Because private addresses have no global meaning, routing information about private networks shall not be propagated on inter-enterprise links, and packets with private source or destination addresses -------------------------------------------------------- should not be forwarded across such links. Routers in networks not ----------------------------------------- using private address space, especially those of Internet service providers, are expected to be configured to reject (filter out) routing information about private networks. If such a router receives such information the rejection shall not be treated as a routing protocol error. By virtue of using RFC1918 addresses on packet-passing interfaces (those which generate ICMP error messages) it is a violation of RFC1918. One could, in turn, disable those messages, or filter them, but as others point out, that breaks such things as PMTU-D. Also, those who think their RFC1918-numbered device is not directly reachable solely due to being RFC1918 numbered, are deluded.