How about using a static security source code analyzer and going through all findings? The very good ones, the commercial ones, are free to use for open source projects. That would be real benefit to the project I think.
It's missing the point entirely, their primary goal is to become better developer and to improve their C knowledge, hence the effort being mostly focused on reading and understanding a fair amount of code, including its context. Fixes and improvements are positive side-effects of the original effort, not goals in and of themselves.
One of problem of static source code analyzers are false positive. Soon or later you will have to reading code and understand the context. I assume it's better way to improve your C knowledge because you REALLY must understand the code . And besides that the positive effect are more valuable.