How would you interface between dense mode routers and sparse mode multicast routers?

This is an IPTV environment. There are two geographically separated environments managed by different teams. Normally when we need to push video back and forth we drop it onto a "video proxy", something like a Cisco DCM that won't allow reverse flow traffic; to protect the source network. Problem is this particular application needs to be able to speak unicast bidirectionally back to some middleware and a VOD server cluster.

Complicating matters is the source network is all dense mode, and it ain't changin'. The remote network gear is only capable of sparse mode (Arista). These two networks need to talk. We have the bandwidth to statically join all groups (around 500) if that would help the solution along -- this wouldn't be a bandwidth waste as we are normally joined to all groups anyways.

So, I turn to the brains of reddit. What would you do?