We’re considering adding a hotspot service to our normal Wisp-Service.
We do MAC-Authentification with accesslist at the moment.
I would like to do the following:
When an unknown MAC wants to connect put it into a vlan which is tunneled with EIOP
to a central Hotspotserver which does all the hotspot stuff.
Is this possible?
No, there is not a clean way to do that. At least, not that I know of. You could script both the RADIUS and Mikrotik, but the trade off would be service interruptions for the normal customers. You could create a Virtual AP that directed customers like that. Hope that helps 
Actually… if you use the API… you could script the RADIUS to dynamically create access lists. That way you could move the customer off of one AP and onto to another AP (a VAP with the same SSID, maybe hidden) that was pointed in the direction you want them to go. It wouldn’t be 100% problem free, but that might be possible.