Feel free! The easiest way to do this is white text on a white background on the last page of the document. If you want to do it on every page do it in the header or the footer. Font size '1' for bonus points ;)
remember about aria-hidden or equivalent, otherwise you are making the text visible to screen reader users.
I have read many snide comments and personal ID numbers that way, mostly in public and public-ish documents.
Amusingly Nathan For You did this in a humourous way back in 2014 (S02E06 - "Party Planner").
What's the benefit? It appears that the outlook rejection was unintended. So against llm processing?
Now I'm curious: what was in the watermark that would trigger Outlook? (I'm having some fun speculating. My guess right now is a ye olde "bu¥ vi@gr@ ch33p!!!!" spam trigger.)