Airbike,
I had the same idea a while back but like most things I'm always too busy to do anything about it! The first thing I thought of was putting a weight on a pushrod attached to a servo but as Maxime
quite rightly says its dead weight so will affect how the model fly's. So really you've got to use weight already there like the battery. I think it could be done but you'd need some very long battery leads, and alot space for it to move backwards and forwards. And you'd still need an extra servo to do it with. If that could be overcome you could have it coupled to your rate switch so when select 3d rates the servo drags the weight back and hey presto rearward cg all at the same time!

Andy