Ualaa's guide covers this - in ISboxer I have amended my DPS button so on 1st press it does IWT for melee group only, and 2nd press it does DPS FTL (as normal). This needs another keybind with IWT for melee groups only. So what happens in WoW is:
For melee: 1st press=IWT, 2nd press=DPS
For Casters: 1st press = nothing, 2nd press=DPS

And all on 1 hotkey!