Can you just fork an open-source project of your choosing (or several) with a UI that you think could be improved, and assign that to your students?
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.
