That was my first thought on that tactic, too. When you're back there in first calss for 10+ hrs and inflight is offering you delicious alcohol, it's easy to slip up and forget that you're "jumpseating"..and unfortunately an easy way to end your career.
I suppose it also depends on what your carrier defines as jumpseating. I "jumpseated" on AA to Europe last year, and they gave me a boarding pass for some seat way in the back. My girlfriend (a FA) used a ZED pass, since AA only allows JS privileges to FAs domestically. She got a boarding pass for the seat next to me in the back. The Captain and FAs then put us up in first class since there were empty seats. The first class attendants insisted that I could drink alcohol, despite "jumpseating," since: 1. I was issued a boarding pass instead of a JS document, and 2. I was not listed on the crew manifest. In AA's eyes, in that case, using my "jumpseat" privileges was basically only a easier (and cheaper) way to non-rev. In fact, the only person who even KNEW I was "jumpseating" in the first place, was the ticket counter lady at JFK. And she just used that as the basis for not charging me a ZED fare to non-rev. I filled out no paperwork, just showed my badge instead of handing over a ZED pass like my girlfriend did. No one on the plane had any idea, seeing as how I boarded with a boarding pass like everyone else.
At Southwest, if a pilot flies using a jumpseat document, then he or she can't drink, even if seated in the back. If they fly seated in the back using only a boarding pass, no one on the plane would know or care if they had a drink.
The way the rules are written, it's kinda' gray, but this seems to be the way it's often interpreted.
Bubba