In the referenced message, Stephen Griffin said:

In the referenced message, Jared Mauch said:

...

I think you are refering to cisco bugid: CSCdv47188

-- snip -- If the first entry in the Multicast Border Gateway Protocol (MBGP) routing table is supernet of the destination IP address or the MBGP route exists but does not have the best path, Reverse Path Forwarding (RPF) lookup will fail or return a unicast Border Gateway Protocol (BGP) route if a unicast BGP route exists.

Workaround: Remove the first entry or add a dummy route that is smaller than the first entry. In case of a MBGP route without a best path, change the network configuration to ensure that the specified destination address has the best path. -- snip --

This does seem to resemble the issue, but in my discussion with cisco it didn't seem they intended on deploying resources on the corner cases I described. I'll check to see how the above bug relates to those issues.

Actually, after confirming with Cisco, the above bug is different. Essentially, if a router has two links, and a route in both iBGP and iMBGP with same igp distance, local pref, mask, etc, the unicast BGP route is chosen for RPF, rather than the multicast route. This would lead to using the "wrong" interface. Although it is related in that, if the RPF check was done against the MRIB first, it would solve this bug.