I should probably explain in a little more detail for those who don't use the Baader Hyperion eyepieces....
The Baader Hyperions (the prime ones, not the zoom), have a removable 1.25" nosepiece assembly that protrudes completely below the 2" nosepiece part of the barrel. You can remove the 1.25" assembly but then you change the focal length of the eyepiece. This is a nice feature but you don't always want to do that. The exceptions are the 24mm and the 36mm both of which do not have any lenses in the 1.25" nosepiece assembly. You can also change the focal length by adding one or two extension rings between the eyepiece and the 1.25" nosepiece. Again, a very nice feature to have but using any of these configurations changes the total length of the eyepiece and it can bottom-out in a diagonal when in its longer configurations.
I didn't realize that the Baader Hyperion Zoom was a bit different. I now see that the 2" nosepiece screws on over the 1.25" nosepiece while the 1.25" nosepiece is still attached so the difference in the length of the whole eyepiece is minimal when comparing the 2" configuration to the 1.25" configuration. The view will still be through the 1.25" nosepiece so the focal length won't change since it remains in place (from what I can tell by looking at the product online). This should have no problems fitting into the 2" diagonal in either configuration.
For the prime focal length Hyperions, the eyepiece actually goes far deeper into a 2" diagonal (with diagonal adapter removed) and, to be honest, I thought that maybe the zoom did this also. After looking at the zoom more closely online, it appears as though the 2" nosepiece screws on over and around the 1.25" nosepiece so the overall length of the eyepiece won't change much. Using the zoom in the 2" configuration will allow you to use the 2" diagonal without the diagonal 2"-1.25" adapter so this will allow the eyepiece to drop in a bit deeper providing some back-focus.
When using the prime Hyperions in the 2" configuration, they can get quite long and they will bottom-out on the mirror. This is where the stop ring comes into play. And again, because the eyepiece extends far deeper into the diagonal, you gain some back-focus here as well which is necessary for some telescopes/focusers (my Skywatcher ED 72mm when used with a 2" diagonal, for instance).
I have a screenshot from the Hyperion brochure but this one is for the prime eyepieces. I've never seen a brochure for the zoom eyepiece. This brochure shows a cross-section view of a diagonal with the extended eyepiece in the 2" configuration. This is what I was referring to in order to gain some back-focus.