My go-to in cases like this it to just step up the solvent-molecular-weight scale until you either A) get the desired result, or B) destroy whatever polymer the thing is made out of.
I don't know about ammonia, but isopropyl alcohol seems to dissolve all the constituents of every dye based ink I've ever tried it on, which ought to encompass all fountain pen inks -- and also inkjet printer inks, for what it's worth.