While messing around this morning on my death knight, I decided to see if a theory I had would work. It did, and seeing that I couldn't find any other posts detailing the trick, I figured I may actually have something worth posting. A previous post on this board (New Mount-Through-Walls glitch and Random explorations) revealed that if you wedge yourself between a tight space and mount, you retain your normal collision box. This coupled with the ability to mount through objects by logging off, which was found by Dellion (couldn't find the thread, will post in a future edit), allows us to mount through things by pressing up against them with our broken collision boxes and the logging off and then back on. Below is a somewhat half-assed guide to my potential discovery, which will hopefully be able to explain it better than this is able to.
The guide.
Credit goes to Leonim and Dellion for their respective discoveries.