See http://mod-buildcraft.com/forums/index.php?topic=196.0

Those frames are going to be hard. They are actually encoded as entities, not blocks, and would need further provision to the builder. Not impossible to support, but clearly not easy, and requires some extra design. I'm bumping this for further work.

As a matter of fact, if we go that far, may as well add support for other items, such as carts, robots and so.