How do you find such places? I've been very curious about part time software work but knowledge online seems to suggest that freelancing is the only route there.
Funnily enough, I wanted to find such a job precisely so I could make my own stuff on the side, which will inevitably involve some signifigant open source contributions due to my tech stack. I've thought about simply being a paid member and essentially double dipping, but that route seems even rarer.
In my case I started writing stories of automated farming communes, compiled my writing in to a zine about a better world, had 260 copies printed and handed them all out, got my then dream job at Google X robotics, then ended up handing a copy of the zine to a robotics engineer and philanthropist who now funds my open source work as we push towards continuous crowd funding as our long term model.
Your story will go differently (lol) but what I can say is that defining your vision, building the appropriate skills, and telling everyone that you can what you’d like to do probably helps a lot. You may have to stick with the unfulfilling job for now, but you can try to build an exit strategy.