If the special sites are ones that have quite well-defined IP address(es) (i.e. they are not huge pools of rotating IPs like FaceBook…) then Aliases should work.
Say you have WAN and SAT links, and you want some special things to fail over to SAT when WAN is down, then make a gateway group, FailoverToSAT, WAN = Tier 1, SAT = Tier 2.
Make an Alias SpecialSites with the names of the sites you want to fail over (myspecial.mailserver.com importantsite.com pfsense.org ...)
At the top of your LAN rules, put a rule: Pass, IPv4, protocol any, source LANnet, destination SpecialSites, gateway FailoverToSAT.