Hi,
Install scripts are downloaded from the server, click on "Supported software" link on the left, and navigate from there (same sections as in the client). If you want to experiment with them, copy&paste the source, make your changes, and then run your locally modified version (Tools > Run a local script, or use playonlinux-bash on it).
About the way to submit script updates, I'm not sure...
Ideally, it should be script maintainer that's contacted, because (s)he's responsible for the script, must have the program hence can test the proposed changes, etc. However for scripts written long ago, original author and maintainer may not visit the site often anymore.
At that point other maintainers come in as a fallback, but they may or may not have the program, in the latter case they must be sufficiently confident that your changes will bring improvements, and then trust changes testing on you, to modify anything. It may not be as difficult as it seems, as many script are in obvious need of maintenance.
Now, Application Test forum or Bug tracker... I'd say that in both cases you should be as clear as possible, mentioning all possibly relevant information (distro, hardware, POL version,...) what you tried, what you expected, what you got instead, as factually as possible. This is specially true for the Bug tracker (the Bug tracker is a tool to help
us fix things). I would like to say "pick the forum for a larger audience" but not so many people reply in the forum :(
About HoMM3 and Wine 1.3.19, are you talking of this thread:
http://www.playonlinux.com/en/topic-7931-Issue_All_of_my_games_run_in_800x600_window.html
Regards,
Pierre.