OK, you'll probably notice the Obrea isn't much for single track but I did ride it on dirt roads today on an easy spin following yesterdays TT
This might be a little harder with the mtn bikes and you'd certainly want to wash them first but flipping up the cushion of the rear double bed made for a great place to transport the bikes:
I'm thinking instead of bungy cord to the window, which wants to open the window, with a little custom work I could put fork mounts in the plywood that would be totally hidden (recessed) when the cushions are in place. Could get three bikes in for sure with the bed pulled out. If you had the double twin arrangement I bet you could do four bikes this way.
Even if I go with a rear bike rack having the ability to lock the bike inside the trailer, out of sight out of mind has some definite appeal. For one thing "my precious" (as Gollum says) TT bike won't get rained on. HEAVEN FORBID!!!