Minetest upstream states that their own fork "IrrlichtMT" should be used to build games-action/minetest. Using upsream Irrlicht is of course still possible, and Minetest compiles and runs just fine doing it this way, however it is missing a variety of patches the Minetest's fork has made (such as fixing clipboard support). So, although using upstream Irrlicht doesn't cause any serious breakage, giving users the option to use the Minetest fork would probably be a good idea. I feel like the best approach for this would be to provide a dev-games/irrlichtmt package, in addition to an irrlichtmt USE flag on games-action/minetest which allows switching from dev-games/irrlicht to dev-games/irrlichmt if the irrlicht USE flag is selected.
Sounds like essentially same as openmw[osg-fork] and openscenegraph-openmw, although I do want these to be kept to the minimum as it increases maintenance (then again, irrlicht hasn't seen a release in 5 years -- I'd like to know what Marecki thinks about it).
I'll look into this now that I maintain games-action/minetest, I've been getting annoyed by some of the issues with vanilla Irrlicht myself. Won't promise any particular solution just yet, though.
Let's get back to this once Irrlicht upstream (which judging from their repo history is still very much alive) have released 1.9.0, as that is what the MT fork is based on. As things stand now, packaging the latter could cause a LOT of trouble.