I wanted to format a TimeSpan in XAML and differentiate it from a normal time format, something like this 02h30m
After a bit of fiddling with curly brackets and back slashes I settled on:
"{Binding Path=Duration, StringFormat={}{0:hh}h{0:mm}m}"
One step forward, two steps back
"{Binding Path=Duration, StringFormat={}{0:hh}h{0:mm}m}"