if the car is level, you can remove the cap in the trunk to access the transfer pump w/o issues. I personally wouldn't remove it with more than say 1/2 tank. Just becuase it is easy to splash and drip while removing and gas stink is hard to get rid of.
Also, if you are replacing the transfer pump you are in for a treat. There is a connector hose/piece that is only avail from the dealer and is ~$40... IN addition, there is a rubber O'ring that seals against the pressure line and you should ensure it is in good shape (I replaced mine with one I luckily had from an old unknown carb rebuild kit). I just went through all of this when I decided to change the x-fer pump just for General principle
That whole setup is a very strange one and is not the same as the Mk1 where the pump just hangs off of a bracket.
Pic of said setup..
Modified by sdezego at 10:41 AM 10-13-2008