It would be really neat to be able to set the background as a custom panorama, much like Custom Main Menu allows.

I hope you'll consider it :)

That is currently on the table as far as planning goes, just need to get there

Awesome, thank you :)

Actually, this should already be possible. Just enable the vanilla panorama, and use the resource pack to override minecraft:textures/gui/title/background/panorama_x for [0-5].

Oh, sweet, ty :D