I'm looking for something to do this as well. I've got the center point of my KoTH map raising up as it unlocks, unfortunately that means everyone falls through as they are killed on it, which doesn't look that good.
I've tried parenting a func_brush invisible block to it, but it has the same problem. I also tried having an invisible func_brush disabled at start and enabling it as the point is at the top position, also to no avail.
Any ideas? Would a different tool texture do the trick? Maybe something other than func_brush?