OK, here's my response based on what I know about the subject, which I'm sure is less than some of the regulars around here.
1. Competitions you see on TV are pretty much always open level competitions. I don't think I've ever seen a syllabus comp televised. At open level you're allowed to dance non-syllabus figures which is why you see all that fancy stuff at high level comps. I'm guessing that the ISTD does have rules regarding what can and cannot be done at the open level, but I couldn't tell you what they are. There are a few syllabus figures that can be considered picture lines, so they're actually allowed starting in syllabus, with some restrictions.
2. Let me check my notes. . . . I have from waltz the Left Whisk, Contra Check, and Hover Corte; nothing in foxtrot; quickstep's Hover Corte; and from tango the Oversway and Contra Check. That may or may not be a complete list of whole figures that are picture lines, but those are the only ones I specifically wrote "picture line" next to in my book. One thing about doing these in syllabus though is that you often don't have much time for them so you can't stretch out the figure and take an extra measure or two of music to develop the line. You're stuck with doing say a Contra Check in one measure where the line's there and gone again instead of stretching it out for two or three measures and slowly developing the line. As for borrowing figures from one dance for another, you can't do it in syllabus competitions, but if you're doing open level competitions borrowing figures is one of the first and easiest things you can do. Social dancing, no one cares, do whatever you want as long as you can lead it. 
3. The picture lines are in there, they're just not specifically called picture lines. The figures noted above are the ones I annotated as such in my ISTD book. The IDTA book is pretty similar, so offhand I doubt it has ones the ISTD book doesn't. Sometimes you can sort of get a line out of part of a figure and I didn't really label those -- I've been told you can take a tango Rock Turn and turn the first step into a lunge-type line (we usually do a Natural Promenade Turn into the Rock Turn) for example.
Hopefully you'll get some other replies from people with more experience in this area. I'd be interested in filling in my own notes on pictures lines, too, so post away people. 
--
James Marshall
marshall@astro.umd.edu
http://www.astro.umd.edu/~marshall
