I do not see the problem (personally), and I do not think its a disadvantage coming from a small area. I'm an east coaster, and work at a little MA area (only 240 vertical feet). Now, for every exam and clinic I've done with PSIA I've had to travel at least 2 hours (typically at least 3 or more) to go to areas with more substantial vertical, pitch and terrain. However, chances are that people going for higher level exams will also take ski vacations or weekend trips to ski at more substantial areas. The difference comes in who continues to train themselves while on these trips?
Now, in terms of terrain parks being used in exams, I'll only say its about time (and something to keep in mind is my area got rid of a run down half-pipe a few years ago, and only recently, last year, re-established a bit of a park).
I basically self trained in preparation for my level 2 and 3 (at an area with only 240 vertical feet mind you) and I'll be the first one to say, the limitation does not come from the terrain or the area(s) you ski/train, but the limitations come from how you train. Just because you do not readily have access to some terrain doesn't mean that you cannot train/practice the skills needed, and if staff trainers cannot train you on these skills, do the research, dig around, and train yourself, who knows, you might even hit a point where you're able to "train the trainers" - which by the way is a very good way to train for level III.
Afterall, the motto for level III is ski any terrain, any condition, and ski it well (not just any terrain at your area).