There's a few aircraft out there with fuel dumps. I believe the iFly 747 has one (hidden somewhere in the thousands of active switches) as well as a B-52 model that, for some odd reason, I randomly found in my Aircraft folder. Must have come as AI with some random scenery, as it had no sound (I modified that with eight 707 engines

I have to turn my speakers waaaay down when I go to spot view.) Theres also quite a few other large planes out there with the feature, but it's pretty rare.
Either way, you can always press ALT-Aircraft-Fuel and Payload-Fuel Quantity and adjust it from there, any place at any time. Personally, I just use a calculator and compute fuel burn for a long run to get in the correct weight and balance ballpark upon landing. It's more realistic and it gives you something to do on a long flight.
low and....well....just low.