Are custom expression builders neccessary?

[Hillbilly]

Does the framework support returning a string from a method such as
GetThemedHelp() which generates and returns the value of the CollapsedImage
property which cannot be known until the page is compiled.

Which event handler should contain the method as written and what is the
expression syntax? The following does not function when the method is called
from the same class as the page.

// GetThemedHelpIcon() string not returned
<cc1:CollapsiblePanelExtender
ID="Help"
CollapsedImage="<%# GetThemedHelpIcon() %>" />