Is it possible to aggregate an issue-level custom field in a project report?

We have a project report that is currently summarizing project-level custom form information per project, and to that we're looking to add an average of a value in a custom field in the issues for those projects.

Is this even possible? I've been digging through the documentation and community posts and it seems possible using the {issue}.{DE:<insert custom field name here>} syntax in text mode, but I can't find a way to do this specifically and my experimentation hasn't worked out. Is this even possible, and if so, what does that syntax look like (assuming it has to be done in text mode).

Thanks in advance!