OK, I’ve tried my above idea and it doesn’t work. Although I’m not surprised, given that I’m mixing bits of code from different releases.
I got hold of 1.1.0b (from Google Code, doh!) and took out the new gallery templates. I’ve installed these on my current 1.0.2 install and rejigged the info in my posts. And no it still doesn’t show descriptive text.
Checking the HTML, it appears that the new template is working, its added some <span></span> tags where the descriptive text ought to go, which does correspond with the new template, but there’s nothing actual data in there.
I’m loathe to update my base working copy of nggallery right now, so will have to wait.