With support for XS integration of iOS storyboards about to make the Stable stream, I would love to be able to use this feature in conjunction with MVVMCross.
Storyboard support is now a part of MvvmCross. Use the one ViewController per Storyboard approach as described in Geir's answer, setting the Storyboard ID, and decorate your MvxViewController partial classes with [MvxFromStoryboard]. See sample code on my blog.