If a candidate feels that there is a very good, publicly available demonstration of their knowledge (be it a project they're involved in, some open-source software, or participation in a technical forum or mailing list), they already have the ability to simply point that out to me in their cover letter, or other communications.
> It's meant to provide another dimension to assess the candidate
This is precisely the problem. It creates an additional dimension, but one that exists only for a subset of candidates and isn't easy to evaluate fairly. That seems likely to create an imbalanced hiring process.
Why should I pay for that? (or implicitly ask the candidate to pay to give it to me?)