This is probably completely moot, since R5 is so elderly, but I was looking for it and didn't find it, so I'll post it in case some other poor, upgrade-forsaken developer finds it useful.
I've been looking for a way to manipulate the contents of a Rich Text field (RTF) on an existing (previously saved) document via Lotus Script.
I kept running into the issue where anything done to the RTF in UIDocument or Document would be overwritten by the previously saved contents of the RTF.
Finally I found a way to manage it, it's not brilliant, it's not elegant, in fact it's rather dumb and brutish, but it works, and "works" is a pretty compelling argument.
Basically, you get a handle to the back-end document by way of NotesDatabase.GetDocumentByUNID -- that is you read the UNID from the document you have, and make a clean request directly from the DB for the document by its UNID:
Set Doc = DB.GetDocumentByUNID( NotesUIWorkspace.CurrentDocument.Document.UniversalID )
"Doc" is now a handle straight to the back-end instance of the document you're working from. Make and save changes to the RTF there, and they will become the "previously saved" data which overwrites the contents of the open Doc's RTF when you save it.