The 8,9, 10 speed designation for rear derailleurs comes from the supposed precision of the alignment of the cage with respect to the cable pull, and the "float" of the guide pulley. 10 speed supposedly has better precision, but I'm darned if I can find any difference between my 9-speed Ultegra stuff and my 10-speed Ultegra stuff. And I've used 9-speed derailluers on 10-speed systems with perfectly fine results and vice-versa. So, if the chain will actually fit through the cage, go for it- get a nice Deore LX derailleur for the big freewheel; the cable pull ratio for the shimano rear derailleurs hasn't changed, with minor oddball exceptions, for years.