Enable MNTP to work with a relative xpath from an unpublished node.

see 

https://groups.google.com/d/msg/umbraco-dev/r9wNbNNXbUM/LME74r29Y1QJ

and 

http://our.umbraco.org/forum/developers/extending-umbraco/38604-MNTP,-multisite-and-published-documents

for context.