The following forum post asks for this feature:
http://silverlight.net/forums/p/46377/124613.aspx.
For now, it should be possible to convert the TimeSpan to a double (ex: TimeSpan.TotalMinutes) with a Converter on the DependentValueBinding. This should accomplish the general goal, though the values of the DependentAxis won't automatically format themselves as well as they could if we natively supported TimeSpan (ex: via sensible intervals, etc.).