Hello David,
Sorry if we cannot get to the problem quickly, but I may need some more information on what you are trying to achieve. I answered to a similar question here:
http://www.telerik.com/community/forums/silverlight/tabcontrol/setting-radtabitem-headertemplate-in-code-with-resource.aspx
There an ItemTemplateSelector is used. The DataTemplates for the selector are extracted from the ApplicationResources but you can get them from anywhere you like, for example create them in code from a string.
Then, you can modify or create this string any way you like. For example you can still keep the DataTemplates in a .xaml file, then read them with an XMLReader (because Xaml is just XML) and modify or create them on the fly.
The ItemContainerStyleSelector works very similarly but instead of DataTemplates, you give styles. You just need to return a valid style, no matter how it is created or loaded.
I will advice to go for a StyleSelector only if you need to change the ControlTemplate or an external property for the containers and it is different for each of the containers. If you need to change the ContentTemplate or the HeaderTemplate, you can use the HeaderTemplateSelector and ItemTemplateSelector.
Here is a simple implementation of a StyleTemplateSelector:
Does this help you in your case? If you are thinking about something different, we may be able to help you with tips or advice.
Sincerely yours,
Miroslav
the Telerik team