Short answer is yes just build a plinth out of 2x4 and WBP.

If you want to replace a tray on a legs with a low profile tray it might be possible if the waste pipe fall is ok (eg it's not too high at the tray end).

You can use a top access trap so it's not necessary to reach under the tray to connect anything. Several companies make them. The most readily available is from McAlpine...

http://www.mcalpineplumbing.com/productlist.asp

I quite like the metal version from Vado (who were previously called Eurobath). See the WG-205xxx range. They do two versions of each size. One has a chrome plated plastic cap like the McAlpine but they also do one with a chrome plated metal cap...

http://www.vado-uk.com/range_products.php?rangeDetailId=101

Fitting would go something like this... remove any chipboard flooring and replace with a WBP base for the tray to rest on at the required height. Mark the position of the waste hole on the WBP using the tray as a template. Cut a generous clearance hole in the WBP for the trap to come through. Before fixing the WBP down install the waste pipe and trap in the floor so the base half of the trap is in the right place and height (eg center of the hole and flush with the top of the WBP). The pipework and trap should be loosely supported so the trap can be moved sideways a little to make alignment with the hole in the tray easier later. Test the pipework for leaks? Then the tray is fixed down in the normal way checking that the trap is roughly centered under the waste hole. Sealer is piped around the waste hole and the top part of the trap screwed through the tray into the base half of the trap. Then the removable cap is fitted. The trap can be cleaned by removing the cap and the inside of the trap comes out through the "plug hole".