Before posting on this forum, I chatted with Asus support, who also found no choices, and escalated the question to their next level.
Apparently it’s necessary to select “13th gen (K/KF)” or “12th gen (K/KF)” to see the choices – which is strange because Intel shows the i7-12700 as supporting ECC, and the W680 motherboard user manual doesn’t require K or KF (at least I didn’t find it).
Also weird is that the user manual claims ECC support on the specs page, but later in the manual says: “You may install 8GB, 16GB, and 32GB and unbuffered non-ECC DDR5 DIMMs into the DIMM sockets”.
In any case, thanks for showing how to display the list.
I’ll contact Asus to see if K/KF is really a requirement.