Plenum cable is also a major PITA to terminate. The jacketing is tighter and stiffer that the usual plastic jacketing.
If you're set to do this, I'd recommend terminating the plenum cable at a jack, then using a regular jumper for the final stretch (at each end). As soon as you start to wiggle the plenum cable (it's not meant to be mobile at all), it's going to come out of the RJ plug and eventually break off (it's solid conductor wire, and you'd have no strain relief).
If you're worried about the toxic fumes thing, then you'd be looking for "Low Smoke, Zero Halogen" type cable ... a notch above plenum. Plenum cable will still burn, it just has a higher flash point. If it burns, it'll still put out some nasty fumes.
Good Luck
Scott