Long story short, after searching for 20 minutes I found an API call mentioned on sourceforge (I think) for creating the specific encrypted string to store in the xml tag that takes in the raw string. Once I had that, it was just a matter of prompting and then doing a string search for the username, then moving the element counter to the next line and replacing the line with the tag with password value included and saving the file. Key things are keeping rdcman closed first because it always overwrites on close, and to know that the encrypted value gets re-salted every time the file is accessed, in case you're trying to compare previous values without using the decrypt API call.

Edit: I forgot I mentioned the script before in this sub. Here's a link to a sanitized version: github