Seems that, because of the way Nether Portals are created, it will not care where it will generate, just as long as there is a solid surface. Solutions involve:

• Generate the portal only on top of Glitter Grass or Grass
• Generate the portal only above Y-level 64

Where to implement said answer is the next question. Is it determined by the Portal Block or Teleporter?

So the twatting commit message didn't close this issue via 6a38434
Closing this.

It would seem that it may be impossible to generate the portal above Y-level 64 due to a restriction in place that would force it to be below Y-level 32. What I will do in any case is go for the choice that it will only generate on top of Grass, Sand, or Glitter Grass.

As for a "unique" portal... bugger it, if anyone has a problem with it, then they will have to put up or shut up until something both unique and viable is brought up.

Addenum: Perhaps make the portal only usable in certain biome types and only in the Overworld or Gaia.

It will go as follows:
If this is the Overworld, we can only make the portal in Hot, Mountainous, or Dry biomes.
If this is Gaia, you can create it anywhere.

How I will approach this, I'll have to investigate.