ralphs

I would caution that unless it's an explosion-rated ultrasonic cleaner (not all that common), if you use a flammable solution (like your acetone), you should put the solution (and item to be cleaned) in a sealed container (we use glass containers with screw lids) and use a water bath in the cleaner. The ultrasonic waves will travel through the glass.
Most ultrasonic cleaners (especially homeowner-types) are not explosion-proof.

SurferRosa

Why not overhaul the freewheel? It's super easy. Leave it on the hub and use a hammer and punch on the lock ring. It's likely reverse threaded. After it's removed, you can take the cog cluster off the body of the freewheel, remove all the bearings, clean everything, and then repack with fresh grease. Use WD-40 on the cluster of cogs only.

Schweinhund

Yeah, but not by the web site, that's actually true.

greatscott

yawn

grumpus

Kerosene or diesel also work well. I usually scrape off the worst of the crud with a small screwdriver that can get into the corners. A wire brush is also good for pre-cleaning on components that it won't damage. A stiff nylon brush helps the solvent to shift dried-on muck. When I clean a chain or cassette this way I usually finish with dish soap and rinse with very hot water, followed by drying on a radiator or stove; this can shift the last of the fine grit and leaves a clean surface for adding lubricant. When you're done leave the solvent to sit in a closed jar and most of the crud will settle out so you can use it again (or filter with a coffee filter).

Crankycrank

Drop the freewheel in a jar with Odorless Mineral Spirits, let soak and take it out several times and give it a few good spins to dislodge the gunk. Save the dirty OMS as the gunk will settle to the bottom of the jar and can be re-used over and over again making it less expensive and less toxic waste.