How to put the nested render function outside of `process_yaml_for_expansion`?

As I opened a discussion in !37 (diffs, comment 17819) .

Is there any good idea to achieve that? The problem is that some render function will need some context from where it located, for example, (this line)[!37 (diffs)] need to know the repo. Also, sometime I want to reference the course variable, like what I've proposed here in https://github.com/inducer/relate/pull/115 (which might cause cache issue but maybe we will need to do similar things in some other cases).

If that can be done, then we can allow users to add extra render context (or namespaces) simply by import.