It's not entirely clear what your "relevant information which may well be useful to the OP" precisely is, but if it's something that could help the OP solve their problem, or at least part of it, then by all means post it as an answer.
Answers don't have to be exhaustive or infallible. It's perfectly fine to post an answer saying, for example, "I'm not sure what the cause of your problem is, but if it's X, you can try solving it by doing Y. If that doesn't help, try Z and let me know what it says."
Also note that your answer doesn't have to be perfect immediately — you can edit it later if new information becomes available, or even if you just happen to think of some way to make it better.
For messages to the OP that really aren't answers in any sense, you should write a comment instead. Unfortunately, you'll need 50 reputation points to do that on the main sites. (Note that you can still comment on your own questions and answers even if you don't have 50 rep.)
Now, 50 rep isn't really that hard to acquire if you just write a few decent questions or answers — each upvote on a question you asked gives you +5 rep, and upvotes on answers earn you twice that! And once you've reached 200 rep on any Stack Exchange site, the association bonus ensures that you'll never need to worry about it again, since your rep will start at 101.
But if you really don't want to wait for that, well, the dirty little secret is that you can usually get away with posting a comment as an answer, as long as it's a good and relevant comment. Just try to be as helpful as you can, and explain why you're posting an answer instead of a comment, and most people will generally overlook that minor breach of etiquette.
In the worst case, someone might downvote your non-answer (which costs you a few rep, if you have any, but not all that much) or flag it for moderator attention, and the moderators may either delete it (if it's not really that helpful) or convert it into an actual comment (if it is).
However, it's also quite possible that you may (perhaps after exchanging some comments with the OP) be able to edit your not-really-an-answer into a real answer, or you may even find that, in fact, others may find your answer more helpful than you think it is and upvote it. Indeed, in my experience, it's a lot more common for people to be excessively self-critical, and to post perfectly good answers as mere comments, than it is to see answers that should have been comments.
The upshot of all this is that, if you think it could help the OP, just go and write that answer even if it isn't a perfect one. Even at worst, it can't really do any harm, and you just may end up making the world a slightly better place.