Freetz is a firmware-extension (modification) for the AVM Fritz!Box and devices with identical hardware.
Does this fall under PiHole directly on Routers (Tomato, MerlinWRT, DD-WRT, openWRT) - #12 by Rockvole ?
If you haven't seen it yet that thread talks about the hurdles encountered trying to use linux/busybox based firmware on routers. I assume this would apply to Freetz as well.
That's not the same as Freetz already has packages like bash, curl, lighttpd, dnsmasq which can be used. I would like to write a package but I need to investigate more time then. Maybe next weekend I can have a deeper look.
Have a look on my post here:
Although this is for Gentoo, it shines some light on what would be needed in general in order to support new operating systems.