3
Jeff,
This raises an issue that is a bit of a pet peeve of mine. There is no centralized form handler per say. The basic functions are present, but module devs don't typically capitalize on them.
Essentially, for core/module forms that have templates, you can just modify the template. The problem is with modules that don't use templates to generate the forms. Those modules will have to be "hacked" to include your functionality. What files will need to be hacked will depend on what modules you are using. There is no standard model here.
In a nutshell, you're going to be getting to know your modules on the code level to apply this hack.
Wish I had better news for you.
Insanity can be defined as "doing the same thing over and over and expecting different results."
Stupidity is not a crime. Therefore, you are free to go.