Seth, PSI sells the ball point hanging pen, as well as the hanging seam ripper.
The Weaver I live with wanted such a scheme with the crochet needles I got from PSI.
PSI now sell the chain ang magnetic fittings separate.
A small pen like the ElToro could be adapted with a piece of 8 mm tube at the end of the body. Or a "shorty" compact fountain pen could be produced.
PSI can furnish chain and fitting for under $10.