Going through info on Unique Landscapes, Elsweyr, and RAEVWD I've come to understand that while TES4LODGen can generate information on distant land objects, it can't generate the distant land elevation/etc meshes if they're out-of-date due to mods. I've used BAIN to get the correct distant land meshes setup for UL and Elsweyr, but I have noticed a couple of places altered by mods where this issue is noticeable as I approach.

Is there some tool like TES4LODGen which can regenerate this information, and I've just missed it? Or is that impossible / very difficult?

Nope. No such tool exists for landscape LOD meshes. Those need to be generated the hard way via the CS and that's NOT an easy thing to do since its prone to bugs and is extremely picky even on systems it will function on.