I can't say worth regard to Swing-Arm models, but my Plunger nut has a couple of threads inside the pin-hole, enough to fit a 'safety shim' ~2mm, but keep in mind the nuts are slightly dished and similar, but different.
If you're planning to leave it in place I think an outboard disc will clash with the cover.
I put a dodgy ally 'degree-disc' behind the spring against the shoulder of the sliding bush, but found after a while that as the shoulder is only half the thickness of the spring, it started to deform so had to modify a hardened washer for the spring to butt up to (as a spring seat).
( slightly off topic, I had a dodgy spring from a recently discussed supplier, that lost tension after only for years of use but found a better replacement at a swappie- better diameter & length so a good time to do the packer (spring still shorter than the original)
And I took a pic...(nut is obviously not fully in place )
Second pic is the crap spring