At long last, the unused headsculpts for Combiner Wars Hoist (Trailbreaker's pretool) and "Ironclad" (Rook's original pretool) have finally been shown to the public thanks to RoboCon's Facebook:

As can be seen from the above image, both heads were part of proposals for more Fun Pub figures, the above ones in particular being SG Autotrooper (the Offroad mold w/ the Hoist head) and SG X-Brawn (the Rook mold with his pretooled alternate head).
The reason I said "Ironclad" as Rook's pretool is, well, there's a bit of a story in that. See, Rook's original head pretool was actually Brawl, with the pretooled head made to resemble Brawl's noggin as, at the time, Hasbro hadn't yet made the separate Brawl mold that we ended up getting, so Rook being used for Brawl was a "just in case" backup plan if Hasbro needed it. They didn't, since Brawl did get his own mold, which left the Brawl-headed Rook pretool unneeded.
Then, when Hasbro was making plans for their Computron giftset, they brought back the Brawl-headed Rook to see use in that set as a new Technobot named "Ironclad", who was gonna replace Afterburner/Afterbreaker since he was gonna instead be made from the Legends class Groove mold (much like how Rook had replaced Groove in Hasbro's Protectobot releases). But then TakaraTomy went and made an Afterburner retooling from their Unite Warriors Deluxe class-sized Groove, so Hasbro quickly got their hands on that and scrapped their plans for both Legends class Afterburner/breaker and new character Ironclad.
Still, the Brawl-headed Rook pretool was not completely forgotten, as Fun Pub used it as the inspiration for the SG X-Brawn character who appeared in the final Shattered Glass-themed Club magazine comic storyline "Another Light". Based on the above image, SG X-Brawn was evidently one of Fun Pub's earlier proposals for BotCon 2016 along with SG Checkpoint (Offroad mold w/ the Trailbreaker head) and the SG Autotroopers also pictured above. But since that con ended up being another Beast Wars-themed con to pay tribute to both that series's 20th anniversary and BotCon's own legacy of having really taken off during the Beast Era, these proposals never came to fruition.
As for the other three proposals seen above, they are Maccadam/Alchemist Prime, Liege Maximo, and Solus Prime, but since those three aren't from Combiner Wars molds, their relevance is nil in this thread.