I got a workaround, disabling SMART capability on the drive (here sdb):
Code:
# smartctl -s off /dev/sdb
This setting is apparently saved in the disk drive's own NVRAM, so it only needs to be used once. The trick is that it has to be run while the udisks2 daemon udisksd is NOT running. (If I disable SMART on the drive while udisksd is running, udisksd helpfully turns it right back on.) Once this is done, and udisksd restarted, the disk is no longer polled, and it goes to sleep as it should.
But it does seem to be a bug in udisks2, that the udev property ID_ATA_FEATURE_SET_SMART_ENABLED is not transfered into udisks2 to set the SmartEnabled property.