Huh. It worked.
As you can see, I still needed to do coder stuff (the deserialize, the explicit typing), but the hardest part - what Epicor wanted - was that the object to deserialize was just a string.
FYI, yes, I certainly tried this. No bueno.
EDIT: I suppose I should ask support about the original topic - the where-clause that does nothing?
I’ll get right on that… 