I've done it before. Wrap a chain around one of the cogs so that the chain is on the ground on the forward side of the free wheel (direction of forward rotation), make sure you've got a few inches of chain on the ground. Lean the assembly with chain side out against a wall or something like that, put the cassette removal tool in place, step on the part of the chain that is on the ground, and proceed to remove the cassette. I got this idea from
Utahmountainbiking.com. He says:
"If you don't have a chain whip, just use a section of chain to immobilize the freewheel. I just take an old broken chain, drape it over the top of the big cog, then stand on the other end with [your] foot. Works great!"
Eventhough I normally use a chain whip, this fix works great in a bind.