We are interested in knowing if the A5/C5 combo will support the A5 doing the tagging (via 802.1x radius authentication/assignment) while having the C5 just being a normal client.
dot1x doesn’t work that way. If you wanted the A5 to do dot1x it would need to be on the wireless interface and the C5 would need to be a dot1x client (unless thats what you meant by “normal client”).
That would be better then the current situation but is moving more in the enterprise WiFi direction then in the service provider direction. If you were doing it that way, you would end up with a C5 in your customer space. i.e you couldn’t put it on a management VLAN.