Dear Alan, I do not use modern fountain pens, because they are not good, just old fountain pen. I am using temporary Geha 790 from 1950-1960 and Pleikan 400, 400 NN from the same period. The nib of these pens is 14 K gold plated and I clean the nibs very often. I will try to find out something about an iron gall ink.
I think you still need to be very careful with vintage 20th century pens too, especially nice ones. I want to try an iron gall ink too, but I wouldn't leave a pen loaded with it for any length it time - I'd fill it before use, then empty and clean it immediately afterwards.
You can probably get some advice on using iron gall ink at the Inky Thoughts section, at...
There are people there who know a lot more about ink than I do. Good luck in your quest, and have fun