Soup is the original grease remover, and works equally well on axle grease, crude oil and fried chicken. A freewheel soaked in soapy water might not come out clean on the inside like a real solvent, but the chunks of grease that remain are unlikely to be where they should be anymore.

Heavy oil - especially Phil Wood Tenacious - is a very effective lubricant for freewheels. Freewheel bearings have the advantage of never being under a lot of friction since they are only used for coasting. So if you end up solvent soaking a freewheel, let it dry and run as much oil as you can get into it, and remember to do so periodically.