SSRS access for AD-trust *groups* not working
I don't have enough reputation to post comments because I'm new here, but this is resolved for me.

I think the root problem is groups are scoped as "domain local" in the foreign AD but that's probably off topic for here.

I created a new group in the developer.local domain and added users from the customer.local domain as members.  This allows the users to access SSRS as expected.