The only problem I see with your idea is that the oil isn't going to flow down through the center hole (where the agitator was?) like I think you are expecting it to. It will be thrown to the outside of the drum along with everything else.

How about if you use a bag made of filter media instead of a plastic bag as a liner? The oil will be forced through the filter by the centrifugal force, and into the outer drum to be pumped off. All the junk will stay inside. One of the great things about your idea (with this slight modification) is that the oil is forced through the filter so you should be able to use the filter longer to filter more oil, plus it will filter through really FAST!

Heck you should be able to fill it over half way, turn it on, and even add more oil while it is running. The centrifugal force should push the oil through the filter media faster than you can pour it out of 5 gallon jugs.