So I checked in at LAX and gave them my KTN and the supervisor informed me that they didn't have a Precheck line, but that they would try. They put my KTN in and *BOOM* it worked, spit out a Precheck boarding pass.

I went over to the T4 (American) Precheck line and it didn't scan It said "INVALID BARCODE FORMAT" on the barcode reader, so apparently there is something not right about how the barcode is generated for LAX- probably not tested by Emirates IT because it's not a valid Precheck location for them.

I went back to the check-in counter after a long walk back, and with a sheepish look asked for a new BP without my KTN We had a good laugh and they told me they'd now prevent anyone else from trying

I hope that my wandering at LAX (and subsequent loss of 30 minutes in the lounge) helps you all not repeat my mistake.