Chuck, what you're asking for is possible in principle with a custom tracer, but could potentially incur a lot of overhead, especially your first option. Depending on the size and structure of the XML, having an Introscope tracer retrieve it and then parse it to extract the StoreNumber value might have more of a time, CPU or memory impact than you'd like.
The second option seems more palatable, although I'm not sure if you're talking about instrumenting saveStoreNumber() with a tracer or having a tracer call saveStoreNumber().
How about a third option: would it be possible for your developers to add the StoreNumber as a parameter e.g. if this is a servlet, as an HTTP header or cookie? I think a custom tracer could retrieve that pretty efficiently, and then a name formatter should allow you to have metrics separated by StoreNumber.