A possible approach towards your scenario is to edit the ControlTemplate of the RadRibbonSplitButton (read more). Then you can rotate the DropDownIndicator element. I prepared a sample project which illustrates this approach.
Please take a look at it and do not hesitate to ask if you need further assistance.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items