Trimming with weight?

[chuck]

A freind of mine made the following suggestion:

Trim with weight not tabs.

If you had a xx-lb weight that could move back and forth in between the rear PAX and the tail you could move the CG around (and thus change the trim) without adding drag.

If the weight you were moving around was something you were already carrying (e.g. tools, spart parts, or whatever) it would only change the weight by the mechanism, not the ballast.

So ignoring (for now) possible issues like complexity, reliability and danger, what magnitude of efficiency gain might be expected? Would tandem designs benefit more since solo they are nose heavy?