This will only be possible with the upcoming 2.3.3 release. AppStore version will not have it by defaul, you'll need to build a customized HWC with the provided template source code from the Mobile SDK.
To accomplish this (in 2.3.3), open HWC-Info.plist in XCode and change the "Supported Interface Orientations" property to the desired values.
Thanks,
Andrew.