If I've understood it well, you want a splash page (with radius authentication) resenbling the one you have with your IAPs. Am I right?
If that's the case, I'm afraid you can't do it with autonomous Cisco APs.
However, I can think of 2 workarounds (one of which i am not too sure of).
- I am not sure if this would work, but maybe you could use your Aruba Virtual Controller as default gateway for the guest vlan. You'd then need to attach your cisco APs (same SSID/encryption) to that vlan and pray for the IAPs splash page to appear.
- You can also buy a wireless LAN controller (both Aruba and Cisco would suit you) and configure a guest portal both for wireless and wired access. Then you'd only need to configure both your WLC and autonomous/instant APs with the same SSID and encryption.
Hope this helps
Regards