Considering that the cameos prior to the Eternum shoutout have been Daz3D models that the CS dev could readily borrow if he asked nicely, I'm not sure that's feasible with the Eternum characters given that they're HS2 models and that would be jarring to see side-by-side with Daz3D renders. So the CS dev will have to remodel the Eternum characters to Daz3D for them to fit, which by itself would be time consuming and the results might not even be worth it, so there's that.
Of course, the other option would be to remodel the MC of CS to HS2 and have that model interact with the characters inside the game in their OG models. But that in itself poses new problems (i.e. dev will have to learn how to use HS2's studio, how to take screenies, scout for HS2 cards that bear the likenesses of the Eternum cast since the cards aren't available to anyone yet (though I suppose he could kindly ask Cari for it), that kind of stuff).
Sooooo...yeah, I can understand why the CS dev would choose to just do what he did for the Eternum cameo (where you could justify the Idriel poster being a representation of the ingame graphics/art direction).