Maybe using a solvent to soften up the plastic would help? I believe acetone is effective on ABS plastic but I don't know what your dust cap is made of.

EDIT: I see CliffordK beat me to it.

Another suggestion to speed things up might be a Dremel tool with an appropriate attachment. Maybe one of the wire wheel attachments would abrade away the plastic without doing too much damage to the threads (but I'd still be very careful about letting the tool touch aluminum for more than an instant)