While looking around for captive spacers for the 300, I noticed that
Fast Frank Racing has a Rear Axle Pull Cup available for the RC390. Pull cups certainly aren't a necessary item, but they do seem awfully convenient. At least enough to justify $30 or so of disposable income. So I inquired about the chance of them fitting the ninjette.
The bad news: the RC390 has a 15mm rear axle, so they won't fit as is.
The good news: Frank (who responded himself, based on the email signature) sounded willing to up the bore to fit our needs.
I sent him a measurement, and we'll see where it goes. His initial response didn't indicate what effect it would have on price. If anyone else is interested, I'll report more as it occurs. If not, uh, sorry for wasting your time I guess?
Update: 2mm larger at the axle and the flange, so it's more of a custom job than anticipated. That brings it up to $45 plus shipping, but I'm apparently at the end of a multi-week queue.