I was discussing this once with the maintainer of an MIT licensed scientific software. The solution in the end IIRC was to require all first-time contributors to agree in writing to a Contributor Agreement that states (among other things) that all code submitted to the project would be licensed under the project license.

It's not much of an obstacle really, just whenever someone submits a PR for the first time, say "I'll look at your code and comment anything that may need improvements. Also, you have to agree to these conditions [link]."

Is this a suboptimal solution?