Hi Natalie,
This can not be done with AtoM's OAI Repository module at this time.
We did have an institution reach out to us earlier this year for a quote on developing this. After a lot of analysis, it seems the best solution would be to add new custom virtual sets for every repository, so those sets can be used to limit the output.
Unfortunately, this would be a major development project, and the total estimate came out to about $29,000. The institution in question did not proceed with sponsorship, and so at this time, the development is not on the roadmap.
If your organization might be interested in co-sponsoring the feature, feel free to contact me off-list, and I can put you in touch with a contact from the original institution to see if they might be interested in cost sharing.
If you had a way of making use of the returned JSON data, AtoM's information object API endpoint includes all the filter parameters available on the search/browse pages in the user interface. See:
You can also export search results as a CSV, or use the clipboard, or use some of the parameters available on the command-line exports. For example, we provide an example of how to do a bulk EAD XML export limited to a single repository in the documentation here:
Regards,