Ability to set RegionalSettings via CSOM is now available in Dec 2014 CU.
See here for an example: https://github.com/OfficeDev/PnP/tree/master/Samples/Core.Settings.LocaleAndLanguage
Unfortunately it appears that only the Locale (and not other Regional Settings properties) can be set after DEC 2014 CU. I'm surprised they went to the effort to update the API and only addressed 1 property...doh.