On the cover of Kazuma Keneko Works 1, there's a Bael design that we never see in any of the games, and is easily my favorite design of him compared to the SMT 1 and 2 designs. Does Kaneko own the design, is that why we've never seen him?
So this familiar face is part of the KKW1 cover art like you said, which is named "Advancing Army." My original printing of KKW1 gives a copyright of "Kazuma Kaneko/ATLUS 2004" but I think it's safe to say Atlus owns it entirely these days. I would have to agree it's Best Bael, even if it is basically just 1992 Bael done in "modern" Kaneko style.
Thing is, Kaneko over the years spruced up/slightly changed the designs for numerous demons appearing on these one-off illustrations.
Here are Berith and Eligor from the "Devil Hearts" artwork done for Comickers. I love the changes to both here. Berith's darker armor and cape give him a little more dimension, and Eligor gains a bit of fashion sense with the fur collar. These two (and Bael, of course) are probably my favorites of the spruce-ups. I forgot these Goetic horseguys' redesigns also appear on this Assembly of Nihilo artwork:
And one more, something very minor:
Satan's jaw on the KKW2 cover art, "Advent" has a separated mandible the original doesn't.