The spec sites for 7640U and 7640U PRO do not differ at all with regard to ECC support. Both say: “Yes (FP7r2 only; Requires platform support)”.
I’m in the same boat as @resample and would risk buying ECC SO-DIMMS just to experiment with this since I would love ECC in my AMD Batch 1 FW 13. The only reason for me to not even try would be a definitive statement with hardware based reasons (other than “it’s the non-pro processor”) for ECC not being able to work, e.g. missing traces on the mainboard.
My desktop is an AM4 board with 3700X bought very close to release and the ECC support needed some UEFI/AGESA updates and patches to the linux kernel to actually work including error reporting. That took several months to come together but since then works perfectly. So if the hardware is at least theoretically capable I will try this. Probably with two of Kingston’s KSM56T46BS8KM-16HA. 5600MT JEDEC and ECC.