Modifying Admin Forms via a plugin

I’m wanting to build a plugin that will intercept the creation of several admin forms so I can customize them. I’d like to use files on the filesystem for templates in place of the templates in the database... this makes for easier version control and easier editing.

I’m kinda flying blind here... is there a better way to customize this than a plugin? I can’t even tell where to begin this thing... I can see which filenames are being called by printing out parts of the $ScriptProperties array:
http://rtfm.modx.com/display/revolution20/OnBeforeManagerPageInit

But now what? Printing from the plugin doesn’t do anything that I can tell, nor does returning a value, nor does modifying the script properties...

Anyone got ideas here? Either the docs are lacking or they’re hard to find.

Thanks!