Honestly, I believe the best option would be separate AR. Customizations can be swapped easily and being undead is permanent thing. The same applies to elf options on Forsaken - you were either human or elf before you died. I guess it does not matter in the end, since most of these customizations are for player RP agenda, but I'd find it awkward. As it currently stands, I find it more likely for Blizz to add it as character customizations (it is one of the major themes of Shadowlands) and I think it's more likely to see it on blood elves, rather then Forsaken, because it takes less effort to do so.
Unfortunatelly, I don't think we will see new AR in a while now, maybe at the end of SL, but I expect it later then sooner, if ever. They would also need to make their models interesting, lore is not enough to make successful AR, nightborne pretty much proved that.
Sure, high elves with blue hair looks great. There are some high elf NPCs with blue hair already.
My void elf now finaly has proper high elf look too!