Sorry, for my not responding to this!
I agree this feature would be a welcome addition. I think the custom XForms action <orx:setlocation> is the way to go instead of an XPath function, because obtaining a location can be assumed to an asynchronous operation, which would be very problematic in an XPath evaluator (it would have to block until a response is received or timeout has passed).
We should also think about the value to return when it is not possible to obtain the location. This should be assumed to be quite likely to happen, due to technical issues, lack of user permission (in a browser, OS), or context (http instead of https in a browser). I haven't given this much thought yet, but "NaN NaN NaN NaN" (yeah, that's ugly) comes to mind following the error response of a Number Function in XPath.