On further investigation on this I found the following in the united.com fare rules:
Fare Basis: ZXHK
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">
NOTE -
THIS FARE IS NOT APPLICABLE FOR ANY OTHER
DISCOUNTS.
---
Z CLASS FARES ARE NOT UPGRADABLE TO FIRST CLASS.
NEITHER MILEAGE NOR CERTIFICATES MAY BE USED TO
UPGRADE THIS FARE. PASSENGERS WISHING TO HAVE THE
OPTION TO UPGRADE SHOULD PURCHASE C OR D CLASS PER
THE RULES OF THOSE SPECIFIC FARES. CORPORATE
VOLUME DISCOUNTS ARE ALSO NOT PERMITTED.
---
Z CLASS FARES ACCRUE BASE MILES AND ELITE STATUS
BONUS MILES BUT ARE NOT ELIGIBLE FOR CABIN/CLASS
OF SERVICE BONUS MILES.
</font>
Bolding is my own addition of key points.
THis would seem to settle the issue.
Interesting to see no COS bonus on these fares. Might be worth splitting off onto another topic.
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RichardMEL, UA 1K
A Star Alliance Member.