In your opinion, other than for funsies, what would be a good situation for an open source project to pursue a custom IC vs say an FPGA or making due with a commercially available IC?
That being said, I think the economies are changing. If you don't need a cutting edge process, the tapeout costs continue to come down. The problem that remains, though, is licensing cost of EDA tooling. There is a little bit of competition in that space, but not much. It is growing though, so hopefully that brings prices down.
Your design would in principle have to be an analog design or something that simply cannot take advantage of smaller nodes i.e. MEMS.