Currently, queries with $_internalJoinHint receive a hash key which is identical to the one that a query with no hint receives.

I think it would be safest to not use the join plan cache for any query hinted with $internalJoinHint or any other $ internal stage for that matter.

However, once a user-facing hint syntax is established, the hint should participate in the cache key so that such queries are cached, but each in a different slot.