After adding a custom font into GeoServer's datadir under the "styles" directory.
Checking the logs show that the font is loaded:
2017-03-23 13:57:34,394 INFO [geoserver.wms] - Loaded font file /var/lib/tomcat7/webapps/geoserver/data/styles/test.ttf, loaded font ...
But does not show up in the available fonts list
Linux Mint 18.1 Serena
One persona note, it might not happen anymore on master and possibly 2.11.x
We have a report of this happening on 2.12.x too on list