I made a simple utility that install, and uninstall, the required registry keys to install plugins.
You can download it from here.
1 - unzip "MozRegKey.exe" on Portable Firefox's installation folder;
2 - run MozRegKey.exe and clic on "Install" button;
3 - run plugin installer;
4 - clic on "Uninstall" button and then "Exit"
Test on Windows Xp with the following plugins:
-Macromedia Flash Player 220.127.116.11
-Macromedia Authorware Web Player 2004.0.0.73
-Macromedia Shockwave Player 10.1.0.11
-JPEG2000 Mozilla Plug-in
Please, give me some feedback.
@John: if you like it, feel free to include it on your package. I released it under GPL license.