Day two of the ZConfig saga, and I've made some tweaks to the zconfig_schema2html.py script (adding default values, nicer display of python objects) - see the Zope configuration schema.
On the other front, I've determined that there is indeed an undo-less store in Zope 2.7, the BDBMinimalStorage (it even appears in the above cofig schema :). Unfortunately, Zope 2.7 is currently unable to mount the store. I could fall back on DBTab, but I can wait for the support in Zope itself (saving myself the effort of implementing the mounting twice).
I found some documentation for the config vars for BDBMinimalStorage and I'll copy them over to the ZConfig schema. Or perhaps not. The descriptions will appear in three places then, and that seems a little suboptimal.