In my experience of the last few years working as a senior software engineer in both public and private research institutions, one way to do free software development professionally is this: Find a niche in a research field and try to work in either research labs, scientific computing departments or get a maintenance grant from a foundation (e.g. CZI). Maybe I am lucky, but it seems that FLOSS is generally welcomed and the trend goes to more support. It also provides you ways to communicate on conferences, etc talking directly to many users (researchers).