As I said in the Rally Navigation Mount thread, we just released v1.0 of an iphone App that has an adjustable ODO and a pretty well sorted CAP heading. We've logged some time on the device and think it works well enough to get it out there and continue to make improvements. We have plans for external switches and more features, but for now this does a good job of replicating an ODO (with big safe numbers) and CAP.
<iframe width="560" height="315" src="//www.youtube.com/embed/yxtUJB0SlcQ"...
RallyBlitz - Rally iPhone App
Source: Adventure Rider