Time Machine exclusions added via tmutil do not appear in TM "Options" list
Aaah, the following page seems to suggest that this is in fact expected behaviour:
http://pondini.org/TM/Works4.html

Exclusions Not Shown on the Preferences Window
Fixed Path Exclusions are set by the CSBackupSetItemExcluded API or sudo tmutil with the -p option, whether the item exists or not.
Sticky exclusions are set in an item's extended attributes, via the CSBackupSetItemExcluded API, xattr, or tmutil without the -p option.