Nope, they never open their legs from crawl and you can't get in there, chains are the only option.

Hmm my girl felt when I opened the chain in crawling pose. Made me happy.

Sorry, my bad, you are right.
I didn't really tried to actually DO anything from that position, only noticed you can get there - you need to jump on the girls legs, then your feet will slide around her legs, or in between them, and then you crouch down and can even insert the penis in, BUT the movement is too restricted due to the leg collisions, so its not really viable to actually fuck the girl in this position.

But maybe the doggy style could be modded in using this crawl pose and loosening up girls legs a bit (adjusting the "springs" toughness via script commands?).

If you release their arms, but keep the legs lock in the chains, you can do a pretty close approximation to a proper doggie style if you try. We need something like puppet mode, which I can't seem to find in the new version

just lock one hand to the chain and the girl gets into puppet mode. I love this new chain idea found in 3.5.

Now, I'm not 100% sure but I believe that it is the deep-throating that actually moves the scene. Make sure that you gag both of them a couple of times and that should work.

Also, I found that if you drop the girl out of the chains, she has a tendency to go into a slightly different doggy pose where you can actually get in there without too much trouble. The are only minor reactions from her though.

The animations are available, so with modding it should be possible to add a new task for this.