I wonder if a Raspberry Pi port would be a "low effort" way to attract more developers.

I'm not claiming that such a port would be easy, but that the Raspberry Pi represents a relatively stable platform. Rather than having people turn away when they discover that component X of their PC is unsupported, they have a path where they can boot the image, try it out, and learn how to develop software for it. While this probably wouldn't address the issue of kernel/driver development, it may help to address application development.