Suggest new subcategory
I guess my thinking was to put all of the subpages into the category. Where it would be inappropriate to have the subpages in Category:OER, it seems right to have them in Category:OER Foundation. However, including the category in the navigator doesn't work if one or more of the subpages is a project unto itself and a sub-category of the main project would be useful to collect all of these pages.
But I agree that structured learning material may be a different animal. Auto-categorising with the OER metadata template is definitely the way to go. I'll add it as a topic on the category guidelines page we're planning so it gets into our thinking.