I would think you could use Kossars as Freeguild Greatswords. They do have great weapons, though maybe not as much armour.
The Knights are a problem, even the Empire Knights only have compendium warscrolls, there isn't any equivalent in the Freeguild currently. As yo say Demigryph Knights are a base size up, and a much beefier unit than horse knights were.
I can understand why they didn't have Warscrolls, since they've only done them for stuff that was on their webstore at the time of launch for AoS. And the only likelihood of them returning would be for a Made to Order, though not many of those have done particularly old models, most have been for the stuff that went OOP just before AoS, and in the first year, when tons of stuff went OOP.