No, you can't point the url of the placeholder to a file from the isolated storage. The url need to be possible to be displayed into the browser.
All the best,
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? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.