Whilst waiting for the spawner block issue to be fixed, I was wondering - is there some format / method / registration of mobs method that's necessary for the AW spawner blocks to be able to see them, user to select and then spawn them?

AW spawner block look into EntityList.stringToClassMapping. Which is the minecraft basis for entity registration. All Minecraft and Forge recommended methods use it. I doubt a mod would purposefully avoid it. That could break a ton of things.
I must remind you that the spawner block abide by the excluded entity list setting in structure configuration file. Default here . Those entities should not be available in game.
No other filtering is performed.

Ok, understood.