I use the garmin 496 for W&B. I have never checked if it can auto adjust for elapsed time/calculated fuel burn.
For long trips, I put in all the data, minus the fuel, and this gives me the W&B for low fuel. If this is within the envelope, and adding fuel would not put me over gross, I can go.
Not sure if one of the EFIS's can display the W&B real time, but it would be cool for maybe once or twice. Then I probably would not use it again.