You'd have to make blocks that were single dimension on their collider size but larger on their visual size; the embers are a good example of this in vanilla. So yeh, it's technically doable, but then you'd have to do it for every permeation of every block combination, which would suck for Gazz. =)
...the left bottopm is the half block, the left top is a normal block that would "float", and the right is a 1.5 block with a 1.0 collider (green) that would also "float", but bleed into the empty space on the half block.
- - - Updated - - -
...or, they could create the blocks through code (all of their paintable blocks seem to be code generated anyway), so... yes?
- - - Updated - - -
Trick is, you would either have to put the collider on the half block as a full block to prevent being able to interact through the middle part, or put the collider as a full block and then when people stand on it they would float. Lose Lose.