See http://mod-buildcraft.com/forums/index.php?topic=310.0. This can be done either through a blueprint option (don't clear air blocks) or by providing a special block not built by blueprints.

It's very unstable too. Generally speaking, it's a bad idea to push a build before it's been formally released. See for example #1620 which is worrisome.

alright, i'll wait then

all stuff that you added recently sounds very cool, would you mind if i make a dev build and put in intro the modpack?

Using a block would have been more useful.
I used to do this using ice blocks in creative in the old versions. For example I had a blueprint for making a railroad, which I wanted to place a redstone torch below the powered rails. I didn't want it to replace anything below the rails except the redstone torch, but I did want it to clear anything up to 3 blocks above the rails in case the railroad went through a mountain.

@psxlover I'd say it's a different capability. But I see what you mean and I agree it's a useful complementary functionality.