How do I place a structure that is partially burried that also has terrain matching?

I know you can use the new 'feature' feature to offset placements, but it doesn't seem to offer the terrain matching option.

What I have is a 25x25 garden-like structure, it has a hidden dungeon underneath. I really want it to blend with the terrain, so I love that feature from the structure element - but you can no longer add a placement condition to offset the Y (am I wrong?).

So I either need to find a way to add terrain matching to a 'feature', or Y offset to a 'structure'. Can anybody point me in the right direction?