A couple of our Mailchimp lists are made up of both subscribers that are in Altru and subscribers that aren't. In addition, since OIC is unable to update organization records in Altru, we find that our Mailchimp-to-Altru formulas often contain records that can't possibly have a match in Altru.
Therefore, it would be nice to be able to configure those formulas to consider only those Mailchimp subscribers with a specific Tag or that are in a specific Segment of the source Mailchimp list. We could then make sure that the Segment or Tag contains only those subscribers that could possibly have a matching Altru record.
This is an interesting idea! We researched the possibilities in Mailchimp's API, and the only additional filtering we could provide (currently) is based on Interests. They do not allow filtering on segments or merge fields at this time. Would the Interest filtering be a suitable option for you? I can see how it may be beneficial to other organizations and how they use Interests, but I'm not sure if it works for the use case you described above.
Filtering on Groups (Interests) should work for us, since we can use a hidden Group to segment out those MC records we don't want pulled into OIC. Please keep me updated on any possible implementation of this idea. Thanks!