IIRC I did exactly that.
I think I replaced the current ones with just some other ones renamed to the same thing. That's not exactly portable but did the job for me... let me go check first.
[root@kindle root]# cd /usr/java/lib/fonts
[root@kindle fonts]# ls -al
Niluje will be able to give you the ins and outs of it all.
But I'm fairly certain I have at some point just jumped fonts in there and renamed them. I don't recall why. for the WAFs testing perhaps.
So if your question is can I dump fonts in there and not trash the system then yes. If it is will this automagically change the relevant fonts, I think you need to run a fc-cache -f -v
see Nilujes comments below: and then restart.
That should do the job IIRC.
EDIT: Hmm yeah I just did this to get huge fonts on the 5 with no Hacks. (well, font ones) That's why I remember it.