I don't have the KT default fontconfig files on hand, so I may be talking out of my ass, but on the K3, I went with another approach for the browser (which also used fc):
The default local.conf used bidirectional aliases (map caecilia to serif, then serif to caecilia with an ordered prefer list), simply putting the custom font on top of those lists was enough. (Cf. local.conf
The downside is I had to bindmount it on top of the default file. (Granted, I don't remember trying doing this in a *new* file, like we usually do in a fc setup, because I already had to bindmount the main config file anyway to put the fc cache in a single writable directory.)
On the other hand, I never really used the browser much, so I didn't test this extensively, so, err, just throwing it out there ;D.