G'day John,
drop me in it why doncha?

Lots of user developed tools, perhaps too many to mention quickly, and all oriented towards the notation side of things of course. My favourites are:
Global_Mod
Parts
Transpose Text chord
MakeChord
MultiBar Rest
and the ever useful Variable Dump.

The names are pretty self explanatory but if anyone wants additional details, just ask.

As for MusicXML, there are a couple of options, but the most promising opportunity in this direction is the implementation of the nwctxt format. This is an export file from NWC2 that contains a textual version of the complete NWC work. It is not limited to a single staff. This text file can be manipulated in any way the user wants, using simple things like a search and replace with a text editor to programs designed to manipulate the file according to specific rules. This could easily be used to create a MusicXML transfer utility. I believe there are a couple of users already looking into this. For myself, my meager programming skills aren't up to it.

There is already one MusicXML import utility that works quite well, but this tool does not need the nwctxt file. Rather it was created before that file format was defined and creates the work directly into an open instance of NWC2. I've often used it to import from MusicXML files created by SharpeEye 2.

Normal user tools can be written in any language capable of running in console mode. To my knowledge tools have been written in Perl, php, python, VBScript (WScript), JScript, java, and there's even one that has been compiled into an executable.

At least one user copies to TSE (The Semware Editor, the successor to QEdit) and then uses preprogrammed macros to manipulate the text and then copies the result back into NWC2.

--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!