When you find out, let me know so that I can use it with a Nemesis officer (that is, Sniper class) "of appropriate level" - no, a Warhulk or Fake is not suitable. 
My characters at Virtueverse
Faces of the City
I sure haven't found one. I was forced to use the bosses from the original group, rather than the custom group. At least I was able to set their minions to the custom group.
How do you pick a boss from a custom group comprised of standard bosses?
My problem is this: I want to have a relatively non-level-restricted arc. I am putting in a non-custom boss in a mission -- a Circle of Thorns mage. (It needs to be a CoT mage for story reasons, and there are no costumes that allow reasonable approximations of the look, either.) Each CoT boss mage type in the game is restricted to a particular level range. Theoretically, I'd like to create a custom group containing one boss from each level range, then tell the MA to use whichever one is level-appropriate.
The first problem with this is that the custom group doesn't appear in the MA editor in the boss detail. No custom groups that don't contain any custom characters -- i.e., ones that are just mixes of enemies from the game -- appear in the drop-down for this detail, although they do appear in drop-downs for other types of details like patrols. I assume this is intentional, since you're always picking a particular boss, so if you're picking a standard one, you'd just pull it from the standard groups.
The second problem is that there doesn't appear to be any way to say "pull a random mob of appropriate level from this custom group", in the general case.
Any idea how to work around this? I don't want to unnecessarily level-restrict my mission by picking a particular boss. (I cannot just use the "random CoT boss" pick, because the random CoT bosses also include the Succubus, etc., which wouldn't make sense in the story.)