register_svg_icon
will work for any SVG, it isn’t specific to Font Awesome
So you’d add a new sprite with your icon, like svg-icons/sprite.svg
— the guidance on the spritesheet is the same as Replace Discourse's default SVG icons with custom icons in a theme (so make sure to set an id)
Then in plugin.rb
you can do:
register_svg_icon "my-svg-id
"