I think I have my version running well enough to release. I have never posted source online before, so bear with me.
It is a console application, done in C# (Visual Studio 2010), which hopefully isn't too inconvenient. I had intended it to be used for personal-use only so I have not bothered adding a GUI for it at this point.
As you'll see, the actual concept is fairly basic.
Source is available here: https://code.google.com/p/xray-builder/
Check-out with Mercurial or download binary from the Downloads section.
I'll attach the binary to this post as well.
Hopefully I haven't overlooked anything major. Please feel free to let me know if there are any issues with anything I have done with all this at all, or any suggestions/changes to improve.
Version 1.13 now includes a py2exe packed version of mobi_unpack.py, eliminating the need to have Python installed (again, I think this is ok?).
I have not had a chance to test this on more than my two Win7 PCs, so let me know if this totally fails.
If you have already saved a mobi_unpack location as the default, you can use the new exe by specifying "-u dist/mobi_unpack.exe".
In theory, you should be able to download the binary and start drag-dropping mobi/azw files onto it without worrying about the pre-requisites!