The following advice just in from a Toyota Dealership
Mastertech:

"You can replace the clutch only on the compressor
without the need to evacuate and recharge the
AC system as long as you keep the AC refrigerant
lines connected to the compressor and as long
as there is working space available to remove the clutch.
But be warned. Most of the time when I see a damaged
AC clutch, the snout of the compressor where the clutch
fits on has been galled and hogged out."

Paul Cherubini