How/where can I see the JavaScript behind the "Search & Remove Text" tool?

The Acrobat source code is highly secret, and even if it wasn't, the built in functions are not created using JavaScript. Often, make a tool "quite like another one" is a huge project, written in C, where no part of Acrobat can be used, only the low level plug-in API.

You can't do it using JavaScript. What you can do, though, is write your own script that searches the entire document for specific phrases and then redacts them, but I should warn you that doing it with JS is much less efficient and fast than using the built-in command, so I would avoid it if possible.

Regarding the list of items: The Actions in Acrobat are defined using an XML file that contains all of the commands in them in plain-text.

The file type is "sequ" (for "sequence"). You can export an Action to a file, open it using a plain-text editor and then see the code in it.

If you do that you'll see that the list of items to redact is easily identifiable and can be expanded using relatively simple means.

After editing the sequ file simply import it back into Acrobat and you're ready to go!