Well, I had some time to mess around while waiting for an answer and discovered it on my own. I made up a plunger (on a Unimat 3 that I also have) and that sucker won't go past that spring in the bull gear without removing the retainer spring as far as I can tell! Maybe if you chamfer the front enough it may, but then I don't know how effective it'll be in the pulley. Might have too little "bite" to be effective.
Thanks anyway, and at least I've now contributed something to the group however uninformed it may be.