A seminar is plenty of time for an itch scratching project or two. The kind of thing where a few lines here or there make a big difference in the usability of the software, even it if doesn't make a big difference in the
functionality of the software.
Furthermore a lot of opensource work is done as someone fixes just one thing about a program that causes them trouble, but the submitter does not become a regular contributor. Certainly that much can be done in a the length of a semester.