I’m a native speaker and the above problem is not very important, but interesting to me. Also I’m almost sure that you can, but want to hear other opinions on it.
I think that it is correct because in spoken German you can leave out es at the end, so spoken werf would correspond to written werf’, which resolve to werfe (subjunctive I). Subjunctive I in German is used e.g. to give instructions, mostly in 3rd person, like “Man nehme …” in cooking recipes.
Knowing this, one could interpret the spoken werf as short term for “werfe er”, adressing the other person in 3rd person singular, which feels oldfashioned but valid to me. Since imperative and instruction share the meaning (maybe with different strength), both choices seem correct in spoken German. (Note that this could have been applied to most other words with irregular imperative in the same way.)
Do you agree or have I made a mistake?