A Platinum Preppy - works fine -it has an o-ring and the thick silicone grease on the threads.

I have a couple of Indian eyedroppers - Serwex 101 and Camlin 22 - no o-ring, just the gunky silicone grease - worked fine, once I learnt not to heat them up next to my body.

I also have a Sheaffer Skrip Cartridge pen with a translucent blue body that I am going to try - haven't inked it yet.

I am very reluctant to do it with any pen that does not have a clear or translucent barrel, for fear of unscrewing it in a forgetful state.
HAS ANYONE ACCIDENTLY DONE THIS?
I also like to see the ink sloshing around in the barrel.

Another comment - I thought the pH of some inks was such that contact with metal barrels was not recommended.
Have I imagined this? I do realise that nibs are metal, so maybe this is a red herring.