Can the valve be removed from the tank?  I haven't looked at mine in a
while.  I seem to remember a big nut behind the knob that will unscrew the
whole valve from the tank.  If this is possible it might allow you to
salvage the valve.  Still would want to soak it with penetrating oil
first.  Once you get the whole assy out, you can soak the threads with oil
from both sides and work it loose.  Only try this once the tank is
empty!  BTW, I think burning the propane is more friendly to the
environment than just letting it escape.

Edward