just get some really long wood-screws and hey presto...
direct mounted pickups..
some people cut away the pickguard or over size the pickguard screw holes (my preferred option) so that the screws dont make contact with the plastic.. no-one will ever bea able to tell you modded the guitar uless they try to remove the
scratch plate 
...
Personally I think any benefits to sustain you gain from direct mounting on a floating guitar are minimal... and not as obvious as using lower magnetic pull pickups or blocking the trem...
Sometimes I go a bit mad and think I can hear a tone difference but I recorded and A/B'd a guitar I did this to once and couldnt hear any difference... also worth mentioning is that once youve set on a pickup height I like to 'pack' under the pickup with slivers of wood or paper so that the pickups are held tight against something thats held tight against the body... I like to think that having a more rigidly held pickup transfers the vibrational energy from the body to the pickups better. Though this more than likely makes no difference to sustain or tone I still like to do it to all my direct mount guitars...
Since generally you wouldnt be using a floating trem if you didnt need to then might I suggest your first step to be check you got your pickup height at a sweet spot that isnt too far away so that you can get all the tones that you want and is as far away as possible to reduce string pull from the magnets.
