You communicate a lot to potential hires about your company in the way you recruit. I would opt myself out of the hiring pool if I noticed a company had a practice like this.

"Do some free work for us and we'll think about if we want to pay you for more" is a dystopian way of hiring that expects the job market to provide a perfect fit with no investment by the company.

I would wage salary suffers somewhere that is focusing on this type of recruitment, because it's focusing on the non-growth side of the problem. Focus your energy where you can build yourself and others up to find success, especially in hiring.

I think comma.ai, George Hotz self-driving company is doing something like that. He has previously posted about hiring contributors. They way they do it is that they offer bounties [0] for fixing bugs or issues of different sizes. They also seem to have design challenges which offer paid rewards as well.

[0] comma.ai/bounties

If the job was as simple as submitting pull requests, sure, but there's usually a lot more process and social-overhead that candidates need to fit in with too.

Also tinygrad does this https://github.com/tinygrad/tinygrad

I would never hire someone solely on the basis of a PR.