Is there a way to find out open source projects that want someone to help with their User Experience?
That way they'd all be on equal footing anyway and can compare each other's designs.
Maybe at the end you can have students vote (or combine work, then polish it) and present it to the upstream author to see if they want to adopt it.
Otherwise, I think you'll end up spamming a lot of open-source projects (which are often already overwhelmed anyway) with student work of dubious quality, which the maintainers might not appreciate it. At least if you filter through their work first and only submit it upstream if it's good enough, you're playing the role of a curator and not just flooding projects with mediocre designs.
At the same time, if I fork any repo of my choice -- I don't think we'll necessary work on the most important aspects. Reducing the chances of the design getting incorporated or the students getting any objective feedback to improve things.