Minn,
Just use a battery powered reverseable pump and two fuel tanks filled with water if you want to shift weight that way.

I've thought of using a jack screw arragement to slide a weight back towards the tail (small weight, long travel) to make a plane really tail heavy for some fun in the air.

As for dealing with retracts, if you use light wheels, the CG shift isn't really that much, and usually results in the CG coming forward when the gear is down, which is a good thing anyway, as mentioned above.