Two methods come to mind.
Both are the sort of thing my gaming expert son seems to find in all computer games - legal 'tools' that allow unintended outcomes but/and which are liable to be frowned on 'by some'.
(1) You can send a near-zero public visibility comment.
Add a comment with a succinct & clear description of the problem at the very start of the , and then, after a 'suitable period' (see below) delete the comment.
The system places a reference to the comment in the relevant user's inbox.
When the comment is deleted it is no longer publicly visible BUT the recipient still receives the notifocation.
I do not know how long the 'suitable period' is that the comment must remain before the recipient sees it after deletion. This could be zero time, with the system forwarding the comment as soon as it is completed, or may require a "housekeeping cycle" or similar.
I'll send you a post & delete comment after I finish this answer to see if you receive it.
(2) More subtle (possibly). If you have enough rep to edit other people's answers, you are asked to submit a reason for the edit on completion. This reason is sent to the user's inbox but AFAIK is not publicly visible.
The system requires edits to be a certain minimum size but a functionally zero content edit is possible.