Is it possible to have a champion model in your special issue models in Pendulum 2021? It won’t usually come up since only the storm vulture is customizable, and the champion can only be taken with the rest of the cadre models but I suppose you could swap in different loadouts for your storm vulture.
From Pendulum 2021: “Potential Champion models gained from having the entire Cadre in your force list must be listed in the grayed area in the provided force list.”, this sounds like the champions have to be defined beforehand, but Special Issue says nothing about champions, so I’m not sure.
It’s a bit tenuous, but I think these parts of the Force List and Special Issue rules cover your question:
So, as I read this, you get 1 Champion added to your list for each Cadre included; literally “you write down the Champion model one time, automatically, for each full Cadre.” If you only have one Cadre included, you add the Champion only once.
So, with only one Cadre, you have only one Champion. Taking this text incredibly literally, you are required to list the Champion in the grey area in the Force List, which, considering you only get to add one Champion, would preclude putting it into the Special Issue list.
Now, I would say that, although not strictly RAW, it makes lots of sense to allow players to put the Champion into the Special Issue list, but you still only have one per Cadre to allocate somewhere on that sheet.
I think it definitely makes sense that if you added a second Cadre, you’d be able to do what you want, and have one Champion written in the Force List section and one Champion in the Special Issue section. At that point you can swap the Champions 1-for-1 (based on the other rules), or swap the Champion for a model of its type (squad, vehicle, warjack, solo), or you could even swap the first Champion into the Special Issue sideboard and have none of them in your list.
But, taken strictly (and ridiculously) RAW, it doesn’t work that way. But, considering when this was written, I think we can blame pandemic restrictions on any weird wording issues. I’m gonna guess we didn’t have a lot of playtest games and “try to break the game” proofreading happening mid-lockdown.