Not technically a fountain pen, but the Karetake No 13 brush pen has a refillable converter and takes fountain pen inks.
This is only my second ink, but I am amazed by it so far! It is a vastly wetter ink than the Karetake black cartridge, and it bleeds through quite a bit cheaper paper, at least with the bath that a brush pen provides. My Leuchtturm1917 is robust enough for it, though. So far, I’ve been pairing it with iroshizuku murasaki-shikibu (Beautyberry) in a Lamy Safari. It is a lovely contrast in my opinion.
I’ve been wanting a brush pen for a while, this looks great!
It is a beauty, and amazing to work with. The only other brush pens that I’ve tried are the cheap, bulk art sets. I didn’t realize just how cheap they were until working with the Kuretake. Well worth the price!