Apologies, I solved my own problem after banging my head for a spell. I just needed to surround the varargs portion of the method arguments with curly braces { } to force the arguments into an array. The new consequence clause looks like so:
$explanation.addMessage(1, "%s Quantity = %,d", {$notice.dispositionCode, $notice.quantity});
>From what I could tell reading MVEL's site, they don't seem to support
varargs but it will still work if the arguments are explicitly wrapped
in an array.
Cheers,
Jason