There is no need for more hints, as long as game doesnt get any bigger than it now is. Point being, with only two items in inventory, its not hard to come to conclusion to try it just in case, as I did. Plus, as I said, I actually liked that puzzle in the end. It would only be a bad puzzle if I would be having loads of items in my inventory, and then having to figure out that from all those hundred items, it is specifically the crowbar I should be using.
Edit: Actually, to put in other words. That you have only letter and crowbar in your inventory, is already a hint that you might need to use the crowbar at that point, since there isnt really anything else you can use. So while it might be one of the last things to come to mind to use, since there are only two items, it doesnt take long to get to the last thing that comes to mind to use.