For the firewall to resolve 192.168.1.100 for example, the PTR has to exist somewhere.. Be it unbound via dhcp registration, static lease registration or host override.
Or some other dns that the clients are registering in, etc. Say for example if your AD shop, you should prob be using your dhcp and dns in your AD.
Then just create a domain override for IP ranges so that you can go ask for PTR from your AD dns, etc..
There are multiple ways to skin this cat - but somewhere involves a PTR lookup from DNS somewhere your clients IP is listed.