Azul,
I was asked (twice actually) on Reddit how el glheþ got its ISO 639 recognition. As I'm not sure of the answer, I figured I'd ask here so I can reply on Reddit.
-Txec R
The OldWitt thread about it was here: https://talossa.proboards.com/thread/7977/talossan-iso-639 (https://talossa.proboards.com/thread/7977/talossan-iso-639). Unfortunately the link to the sil.org discussion itself is dead, but I remember the vibe of that was pretty much "sure, why not?".