"Quite obviously, one doesn't want to pump hot air into the atmosphere when the LEs are deployed ..."
I'm not sure what is meant by "pumping air into the atmosphere". The 747-400 WAI system does this anyway after the hot bleed air has circulated in the sealed area of the leading edge at the very front of the wing, irrespective of what position the LE flaps are in (on a non-Auto WAI system). I have the feeling that some of you are thinking that hot anti-ice air circulates in the wing cavity where the LE Flaps are stowed(?) It doesn't. The heated area and this cavity are isolated completely by sheetmetal (for the entire length of the wing LE (where the WAI is)).
http://members.ozemail.com.au/~b744er/WingLE.gif
I suppose some warmth will be conducted via the metal into this cavity, but this will probably be greatly offset by the coldness of the fuel in the tanks. You'd probably get far more heat coming off the bleed air duct itself, which resides in the same space as the LE Flaps.
After the air has circulated in the very front of the leading edge, it is vented c/o small slots under the wing LE just in front of the stowed Leading Edge panels. There are only six slots per wing and they are so small and widely spaced apart, it's unlikely that the vents have any secondary purpose, such as keeping the stowed LE's free of ice. As you can see from the following picture, there is not even one slot per LE flap panel.
http://members.ozemail.com.au/~b744er/LeadingEdge1a.jpg
Anyway, thanks for the great feedback
Cheers.
Q.