I'm doing what you're talking about 100%. I've found that established and popular FOSS web software that has normal people users and their support forums, plural, for their normal users (as any popular software has) - this type of "established and non-sexy probably used for office work software" is fully known and understood by LLMs. It's very simple to put a support AI Agent inside the interface to that software, and because the LLM has both the user support forums and the source code itself in the training, that AI Agent can do things against the software's API and data structures by request - it knows them!
Yeah, "boring software" is no longer boring when it can be integrated with characters that know that software as good as the lead developer who wrote it and have the knowledge of a dozen PhDs too.