At Origins and GenCon this year, I saw several organized play leagues for a number of systems, including D&D 5e, Pathfinder, Shadowrun, and Legend of the Five Rings. I understand that the point of these organized play leagues is to create a character that you can use throughout multiple adventures (kind of like a campaign run at a convention) and most of these have an entire session devoted to creating a legal character (for Shadowrun, it was the "Welcome to the Sixth World" event).
Honestly, I have no interest in keeping a character for multiple modules: I just want to try the game once or twice and then move on. For instance, I wanted to play Shadowrun 5th Edition at Origins this year, but the only events that were run for it were organized play, which I didn't want to commit to long term or spend a multi-hour session making a character for (I just wanted to play one game).
Is is possible to just drop in for one or two games in these organized play leagues and not have to bother with tracking a long term character? Can I just use a pregen like any other con game (and if so, can I do it for any module or just the low-level modules)? If the answer varies between the different organized play leagues, I'd like to know that too.
A good answer would include descriptions for the two most popular organized play: D&D and Pathfinder, and at least one other one (since the first two are so similar). If I can have answers for all of the organized play leagues that I mentioned and any others, that would be even more appreciated.