Utilizing High Availability between an office and Co-Location in data center

I have a disaster recover location/co-location through our ISP and would like to be able to continue operating mail and remote access servers if there were a disastrous event.  Because we have a gigabit link between the buildings, I was thinking we could actually use Active-Passive HA.  Currently the co-location is tied to the local network between the switches on both sides.  If I were to trunk the port on the switch on each end, make a new VLAN for the HA traffic and give each Sophos an access or untagged port in the VLAN for an HA interface – do you think I can enable HA between our office and the Co-location?  The ISP would also need to configure our public IP block on an Ethernet hand-off at the co-location as well as our office.  My concerns are passing the HA traffic through a VLAN on a trunk port even though the line is gigabit but there still would be other VLAN traffic on that link.  Any input would be greatly appreciated.