"Is it normal for 747's to dump fuel onto a taxiway or was this a potential disaster that failed to manifest itself?"
Any fuel spillage is cause for concern, both for environmental concerns and for safety reasons. If there was a continuous flow, then this suggests that something was amiss.
There is an air vent system fitted to 747 tanks which allows for two things:
1. a way of getting rid of air when the tanks are being fuelled. If the air (normally) above the fuel is vented overboard, then it is easier to fuel the aircraft... i.e. there is less back pressure.
2. a way of providing a positive air pressure on top of the fuel when the aircraft is in the air. Air is scooped up from outside via a Naca scoop into the venting system and it is used to pressurise the tanks. A small positive air pressure in a fuel tank allows for better fuel feed and, I believe, helps prevent fuel vapour building up above the fuel (i.e. there is less risk of an explosion).
A network of vent pipes links the fuel tanks to small "Surge Tanks" in the wingtips. Although float valves are fitted to the ends of the pipes (in the tanks) to prevent fuel entering the pipes and flowing to the Surge Tanks, fuel sloshing around in the tanks, say, during taxy, may get past the float valves. Small quantities of fuel which get into the surge tanks simply drain back into the inboard tanks via different pipes (with non-return valves in them). However, if large quantities of fuel get into the Surge Tanks, eventually a limit is reached and the fuel spills out of the Surge Tank onto the tarmac (or wherever) via the Naca scoops.
If, on the other hand, the fuel being dumped from your aircaft was coming from the Fuel Jettison pipes on the trailing edges of the wings (towards the wingtips), then you have, perhaps, even more serious problems.
Rgds.
Q.