Thanks for sharing your thoughts on different strategies. After thinking about this a bit, here's what I'm going to try. Instead of having separate projects and trying to pull content in from a parent project, I'll just have one project that has various folders. In my Jekyll project, I'll simply pull from the content I need because it will all be in the same project.
For example, in a pages folder, I might have project_a, project_b, and project_c. Then I'll just have to filter out content for those projects when I build my output.
One other challenge is that we have several other writers working on other projects. Up until now, they have been handling their content separately from me. Everyone has their own Jekyll project for the different products.
In this new paradigm, we would all be working within the same large project. As a result, when each person gets the latest files from the repo, there will be a lot of files from other projects. But as long as we filter them out when we build our own outputs, then it should work. If everyone works in the same project, we'll be more consistent with our styles, variables, and other approaches.
However, as a drawback, if someone puts some content in the project that doesn't validate (like liquid tags that aren't valid), it will screw the builds up for everyone else. Yet at the same time, others could more easily help each other out when their sites don't build.