What kind of EFIS do you have?
Foreflight has a new feature to do this in the PRO version.
Got me to thinking. I would have to do some sensitivity calcs, but I would bet you could put a strain gauge on each gear leg, calibrate them for a known empty weight and CG, further calibrate them by loading test weights, and then with the signals (and while on the ground) dynamically calculate and display actual loaded weight and CG as you load and get aboard the plane. Anyone ever cobble something like that together?