There is a recently formed netplus-eduroam-admins listserv created by the eduroam-US organization to discuss technical problems like this, and it recently has been having a very similar conversation about required RADIUS attributes, etc. (See https://lists.internet2.edu/sympa/info/netplus-eduroam-admins ) It does seem to me that there aren't a ton of specifics about what exactly are the required attributes for eduroam-US proxying to work correctly. You can google search to find some recommendations from other top-level administrators ( like eduroam-UK at ...
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