I listened to my intrusive thoughts and filled a cheap fountain pen with (dye-based) printer ink.
I hoped for the darkest black, but it actually looks gray when used with a pen :(
But yeah, it works. It feathers and bleeds through a lot on cheap paper, but it works. By that I mean that it bleeds even through 120g/m2 printer paper. Maybe I could try drying it a bit first...
Might look good with cyan.
Although, I guess ecotank will have piezoelectric nozzles since they're permanent, so there shouldn't be a problem with how it behaves under heat, but viscosity (if that's the right word) and particle size...
Probably a terrible idea. I do also have a sacrificial HP in the garage to rip apart and try to implement a FOSS/OSHW controller for at some point. Maybe I'll try with that eventually, supposing I can hack together a nozzle.