Captive portal on different subnet in datacentre, routed network

[d.liebrand]

Hi there,
we have a big network with a lot of PFsense boxes. Today these boxes are on every separate location. but we want to move them to the datacentre.
We use transit vlan's between the core switches and to the datacenter. so no vlan's will be accessible in a different location. The core switches route the traffic. 
Because we dont trunk the vlan's through our network the LAN interface can't be in the the vlan the clients will be. We can use the DHCP function because of DHCP relay in the core switch on the locations.
But probably the captive portal wont come up because i cant force the traffic from the hotspot vlan to go through the pfsense box.

It there a possibility to make this work without trunking the vlans over the datacenter?

see attachment for possible topography

Kind regards