Hi Unisomniac,
do not let the list of tires from the description throw you off. They are here just for information and usually only account for the tire sizes available at the time it is written and not the max size.
Second, as mentioned before, it is all about the height (mostly) and to my surprise when I measured, 26x5 is bigger (in diameter) than 29x3.0.
So in an Oregon/Hatchet, the height will be the limit and you should find the diameter of the 27.5 rim + tire combo you want.
For the 26" fat tire and the Oregon (ancestor of the Hatchet), you will find plenty of working setup with the Surly Bud, Surly Lou, Schwalbe Jumbo Jim and there is one SnowShoe 2XL. There is a dedicated Fat tires thread to discuss and share reviews.
The most standard threads are the Monster Uni, Current projects and Latest Ride. However, like with the 2XL, they can be discussed separately (and only the search function will be able to help you find them).
I can’t wait to see the pics and feedback on your 27.5 fat setup 