Add glyphs to unicode reserved plane

I'm trying to use FiraCode w/ ligatures in a text editor I'm writing (where I'm currently using Deja Vu Sans Mono), but there doesn't seem to be any way to access the ligatures from Go, where the standard font drawing methods are based on converting unicode runes to glyphs and have no knowledge of ligatures.

The Unicode private use area planes is designed for this purpose, according to wikipedia, but doesn't seem to be used by Firacode (at least as far as I can tell by printing runes starting at 0x100000 or 0x0f0000 and visually scanning). Would it be possible to map the ligatures to the supplementary plan? If they were accessible as runes in a private use area with a documented mapping, I could manually do the context-sensitive mapping when rendering.

It's possible that I'm missing something, because all the "how to use the font" documentation seems to be user documentation, and I can't find any developer documentation..

The issue does not identify a repository file, test, or build entry point. Start by reviewing FiraCode's existing ligature and user documentation, then compare the requested supplementary private-use mapping with the Unicode private-use planes and Go's font drawing API. Done would mean an accepted, documented mapping that is accessible to the requesting editor.