Comments are not the place to give a non-IPS suggestion but while thinking out loud whether the non-IPS comment might be moved to chat, you might have found a good and acceptable way to help OP, because 'chat' as defined at Stack Exchange certainly has greater scope for presenting things that are only peripherally on topic or even outright off topic, such as a non-IPS solution.
In fact there is nothing (except their individual "IPS conscience") stopping anyone from posting a non-IPS solution in the general chat section but how to inform OP if the OP is not a regular participant on the chat pages? It might well be possible (though you need to try it out to be sure) to post a non-IPS suggestion for OP in the general chatroom and hyperlink it into a comment under OP's question.
That means OP will be contacted through the comment even if they never otherwise visit chat, and they can then follow the link to the relevant chat message even a few hours later. Most importantly the non-IPS solution will not appear as a comment or an answer in the main page. Will this work in practice? Members please try it out and give your valuable feedback in the comments section!
Update: I tried it out right here as a comment:
Testing... Hello @OP, I have left a possible solution (not IPS) in chat: https://chat.stackexchange.com/transcript/message/41337146#41337146
Useful tip from @Magisch to copy the hyperlink of your own chat message soon after posting:
click the lil arrow dropdown on the left end of your message and copy the link adress of the "permalink"
It works in practice to the extent that you can actually leave a (non IPS suggestion in a) chat message and hyperlink it into a comment for OP that does not display the non IPS solution on the main page, and OP can actually follow the link and read the non IPS chat message.
Will this work a few hours later? I THINK SO. But will this method be accepted by moderators and the community? Stay tuned for downvotes and updates here!
Update 2: as I expected, some users don't approve of this method because
This would basically be leaving an answer in comments in a very cumbersome way, while comments are for asking clarification or suggesting improvements – Tinkeringbell
So I was looking for another way to leave a chat message for OP, completely bypassing the question page, and was kindly helped out by a tip from @NVZ:
There is an option under the "chat profile" of a user to "invite" them to a room or start a new room with them, and they will be notified just like any other notification. They can then join or ignore the room. Visit https://chat.stackexchange.com/users and search there for the required user's name to get to their chat profile, which is different from the normal SE profile.
I have tested this method and it works but some members expressed the concern in the general chatroom that users will begin to 'misuse chat' for this purpose. However it is a nonsense to think that what is off-topic by the strict regulations of the main site will be automatically off-topic for chat, and I have often seen some extremely irrelevant things being discussed on the Awkward Silence. It's OK because chat as used on SE has a very wide scope for expressing ideas and sharing information even remotely relevant to the site and the issues asked here. It is certainly a place to give OP a non IPS tip that can't be posted on the main site.
So thanks for the expert information @NVZ! I am not going to give a non-IPS suggestion to any OP but it is useful to know that there is a proper way to contact a member without posting any off topic answer or comment, if anybody wants to give them a non-IPS suggestion in a chat message.