I'm confusing by what you're trying to achieve here since you're already saying it's okay for the clients to get a warning about the server cert not being trusted. Since you're okay with that, then what's the issue with just having the RADIUS server present its cert to the clients instead of the AP's as you have done with Aruba in the past? I agree with @DCooper that you should avoid having clients manually accepting invalid (to them) certs if at all possible. If you have management of the devices connecting to your WiFi, then you should do everything possible to ensure they trust the cert you will present from the RADIUS server. I do understand this isn't generally feasible in a BYOD setup however.
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