I've had a go at modifying the existing script to search by song myself, and it turned out ok but there are a couple of quirks. (You'll find the modified script attached)
1. MusicBrainz uses an "offset" value for the track number in the XML, instead of the actual track number on the disc. Which means track 1 will have an offset of 0, track 2 will have an offset of 1, etc. This would be trivial to fix if I could just do a single addition operation.
It has no bearing on the final tag that is written however, only on what is displayed in the initial list of albums before you click the one you want.
2. Can't get the preview button to work, because I'm absolutely crap at regular expressions, so if someone knows a way to fix that up by all means let me know.
Would it be too much to ask to consider allowing the scripting system to use multiple input fields instead of just a single one for each script?