If you can figure out the ID of the post you want, this shortcut url will work. For example, for ID 112516967: https://www.mountainproject.com/forum/message/112519404 You can get the ID by editing the post (if you own it) and looking at that url. Or you can press "Quote", get the "quoteId" from that url, and then hit back (without creating a new post). If you know how to use the browser console, there is an anchor tag at the start of each post that has the id in it.
(Updated after changing to new forum code in spring 2017)