Thanks for the comments. Based on them I found https://github.com/snapcrafters/gimp/issues/21 which suggested copying the font to ~/.fonts/ (which didn’t work) and to /usr/local/share/fonts/ which fixed it - Epiphany can now see the font, and I can now set it as default in Firefox (tho Firefox is ignoring it in a page’s CSS).
There are worse things than death, like being successful by screwing people over and/or making the biosphere unlivable.