It wasn't the best, was it?!
The stain has gone completely: about 95% of it went with repeated cotton wool bud soaking, drying and scrubbing from inside the cap and out (I figured since the ink came from inside the cap it might make sense to try and get a really wet bud in there and squeeze it out, so the solution worked its way through the same route as the ink had).
Then the last 5% or so looked like it wasn't budging, so I taped over the end with electrical tape and went gently but rapidly with the melamine sponge, testing every few seconds to make sure it wasn't causing noticeable abrasions or erosion.
It went really well. Although the stain must have been inside the material, there appears to have been NO change whatsoever. I've tried melamine sponge on No Nonsenses and old pattern (torpedo) Osmiroid 65s before, and a similar celluloid Unique to this pen, and in all cases it has worked well, but noticeably, with a slight loss of sheen or depth to the plastic finish, especially with the No Nonsense.
I do recommend melamine sponge as a very very kind abrasive, preferably without moisture. For future reference of anyone further interested, here are my two pearl Osmiroid 65s. The nice one looked pretty much the same as the mangled one: some surface stains, some deeper stains, some light scratching, and some quite extensive bite marks (although the bite marks weren't as bad on the one that's cleaned up here). It even seems to have dislodged the ingrained filth from the imprint without significantly altering the surface of the pen.
Edited by MercianScribe, 11 May 2017 - 14:48.