It's been a while since I played the game, but doesn't he need the key to get the gates open? And for that you need the post. I don't remember if it's possible to get the key without dealing with the beavers first, or vice versa, but I'd think so.
I lost the signpost once too. Do you mean that you can't find a way back to the place where Glottis was first, or is the post really, really gone?
It took me a while before I hit the the right spot and got to the place where the post was. If you've dealt with the beavers, then you must have got some extra bones so you should know where the post is. Or maybe I really have forgotten something important.