To report back on this: yes it works. If you add fumaric acid to acetone with dmt in it, all dmt will precipitate as fumarate within seconds upon stirring.
However, if your dmt is impure to begin with a lot more impurities will get trapped in the dmt fumarate crystals because they form so fast.
So if you have enough acetone, just add FASA. If your dmt is very pure or your acetone is scarce this is a good method to save acetone.