What if you created a buffer table in your external database, that is populated by your field-merging query, and then invoke that from Newscale? The temporary table would be updatable by newscale (although, presumably, you wouldn't actually have news...

Could you test it using a script, and invoke the rule only if the test got the proper results?

Could they simply create a checkbox list for the model, and a separate  set of text entry fields for the quantity, and then make a third, hidden field matching each pair, where the two values are merged, using that as the actual order field? The form...