I am pretty near done my current bike build project, just missing a front derailleur. I can't think of why front derailleurs would need to be compatible with different numbers of speeds, given that the width of the sprocket will be about the same for an 9 or 10 speed cassette.
So will it work? I bet it will.
edit, FYI: I have this installed and it has been working fine with a front derailleur marked as 9-speed for about 6 months.