Is there a way to pass a timechart span variable to a saved search being called from a drop down? Is there a way to pass it strictly through the xml?
You may use the savedsearch function:
No. If you are using a saved search you can't pass variables to it.
However, if you convert the saved search to a macro, then call the macro from your xml with a Search module rather than a SavedSearch module you get the behavior that you are looking for.