When the patch added Glitterhoof as an actual in-game character I'd assumed that the horse trait meant console commands were the only way to interact with him- but, as another user figured out in this thread
, there's a hole in it since it doesn't block him being nominated as a bishop. By nominating him and waiting for the current bishop to die he should spawn some horse culture/ethnicity courtiers of his own (and if you want to be sure of this you can grant him independence)- except they won't have the horse trait. After that you can grant them land and marry these new courtiers freely, so I just bred my rulers with them until I got an heir with horse ethnicity.
After that, I just selected her with elective succession, took the reins of the Byzantine Empire through a civil war, purged all the human rulers to stabilise my rule, replaced them all with horses, and fought a few wars against
this perfectly balanced and historical coaltion
to grab the required provinces to reform the Empire as you see here. This took me something like three solid days of gameplay and lots of planning, but I'm pretty proud of it- especially considering that I'm probably literally the only player to have done this in Ironman. I've got a lot of screenshots from during the campaign, and I'll post those in a more general writeup tommorow!
EDIT: Forgot to mention, the bishopric you nominate him as heir to HAS to be count-level or higher. You can't grant him titles, so can't just have him inherit a regular barony-level bishopric then grant him the county, and baron-level characters won't spawn in other courtiers of their own culture.