Peter Torr - MSFT
Yes, you can use author-defined expressions inside timing blocks of your markup files, but as donbox5 says they can only be used in timing expressions, not in set or animate calls or as part of the styling block.
Remeber to set all your XPath variables to their default values during your markupLoadedHandler function, otherwise your markup will fail on the first tick.
If you just want to use a constant value in your markup, you can look at the pre-processor I wrote about on my blog.
http://blogs.msdn.com/ptorr/archive/2007/01/10/simple-pre-processor-for-hdi.aspx
If you really want to dynamically change the value of an animation, consider doing it via the Animated Property API instead of via timing.