Thanks for the clarification.
Now whatever architecture I come up with, I am always going to have routers between the clients and the captive portal. From looking at the captive portal configuration page, I figured disabling MAC filtering would allow pfsense to support this setup.
Considering our case is not a paying internet solution, but rather a setup to offer separate guest wifi internet access in our customers' offices, I'm not really worried about people sharing logins, as they wouldn't be able to log in simultaneously with the same login anyway, provided I enable 'Disable concurrent logins'. Am I correct in saying that?
Thanks