I think the Marathon Man clip shows what the problem is
Seriously, it looks concealable, but you've got to figure out some sort of trigger that's also inconspicuous, and then keep the whole thing on your arm without accidentally triggering it. It's easier to simply hide a knife in a sheath somewhere.
I've seen pics of guns and dart throwers in that configuration, but offhand I don't know of a real implementation of it anywhere.
Best,
F