The only way i can think of to do it is to create (non-electrical?) parts for the mounting hardware too. Unfortunately you need to manually place those on the bottom side of the PCB. Then a component height keepout on the bottom side will do what is needed.
One thing to watch out for is that when I've done this before I've manually changed the refdes to U5A, U5B, U5C, etc. Later when I added a part PADS saw that U5 was available and used that as the refdes for the new part (oops, my mistake). So now I had U5, U5A, U5B... on the PCB. That caused confusion when the manuals were getting written!
Another thing to watch out for is that keepouts are only checked in the DRC when they are visible. I often turn off my component height keepouts while placing and routing as they are annoying to try and look 'through'. If I don't make them visible before I run DRC they don't get checked and I get into trouble! So I turn them on and run a DRC after placement, and then turn them back off before I start routing, and then turn them back on when the design is complete (on, off, on, off...).