I knew a few people who'd done this, actually.
It's never been that uncommon. It's the big thing now - or at least it was a few years ago, and had been for some time - in Solidarity, the party of Carrol Cox and others. They have a bunch of stuff about it on their Web site.
http://www.solidarity-us.org/radicalsatwork http://www.solidarity-us.org/rankandfilestrategy
I knew some guys who were pretty successful at it New York, if Teamster local politics were your bag. They've built Teamsters for a Democratic Union into a substantial, if not overwhelming, caucus, and also had something to do with the emergence of New Directions within TWU Local 100 through a along-ago influx of radicals (like Elena Kagan's brother).
So they've played a role in two of the more significant strikes in recent American history, at least (UPS in 1997 and the MTA in 2005).
I don't know much about the early history of this strategy, but if you want to explore it over the decades since SDS' disintegration, the Solidarity and Labor Notes sites are good places to look.
-- "Hige sceal þe heardra, heorte þe cenre, mod sceal þe mare, þe ure mægen lytlað."