There was a request on the English Wikipedia's Technical Village Pump to add Asia/Beijing to the list of time zones accessible under Preferences --> Appearance --> Time offset. Given the number of lesser cities on the list, leaving out Beijing does seem like an oversight, although apparently Asia/Hong Kong can be selected to get the same effect.
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The time zone list in MediaWiki is just the built-in PHP list: http://php.net/manual/en/timezones.asia.php
I would say this is an upstream PHP enhancement request that could be filed at https://bugs.php.net/report.php