I personally dislike being spammed on Twitter, which is why I follow very few accounts directly, and push most accounts into a relevent list (that I don’t look at often enough). Although Twitter bought Tweetdeck, it doesn’t seem like they’re interested in building tools for timezones. They do have their marketing subdomain at https://marketing.twitter.com that might be of some use, but personally I would solve this problem in this way:
If you want to cater respectfully to a global audience, create audience/region specific accounts. We have the America’s, Europe and the Far East. So create a Twitter account for Europe and one for the Far East too. This way you could even push event specific community news to the relevent region account (I realize some people do like to see foreign event news, because some people will travel to them).
Hootsuite is your friend. I’m not sure what their paid accounts will give you beyond the free offering, so you may be able to do better than my suggestion which is, for each Twitter account, you can OAuth with Hootsuite and set-up the sheduling window for publishing tweets from that Twitter account. They don’t have timezone support directly, but you can configure it relative to your location.
Then configure each account to publish news from the same RSS feed that you update with Tweets you want published. Hootsuite will then ensure that each account only publishes the Tweet when the publishing window comes around.
It might be possible to republish the @rustlang tweets to the region accounts, as this would be ideal and eliminate the RSS feed (because there’s the hassle of getting @account names correct), but I can’t see that as a feature in Hootsuite Free. But you could probably find a Twitter to RSS tool out there. Plus Hootsuite has lots of plug-in apps.
You could advertize the new location/audience specific accounts initially on @rustlang, and I think Twitter would probably do a good job of suggesting the them beyond that.