Although it required an embarrassingly long search, I finally found the passage to which Basil the Lesser refers in his commentary on Gregory’s 41st oration. The reason for my aporia was that most of Maximus’s Ambigua is not in the TLG, which is where I was looking for it! The Ambigua is a massive, sprawling work devoted to ambiguous passages in Gregory of Nazianzus. It’s divided into two parts, the Ambigua to Thomas is in the TLG, but this only includes 5 “difficulties.” The rest of the work, the Ambigua to John is much larger (well over 100 if the text I found is accurate), but is not in the TLG. Of course after discovering this the hard way, and finding the text elsewhere, I discovered that the apparatus of the Sources Chrétiennes pointed me to the Patrologia Graeca vol. 91. I’ve learned a lesson though: don’t over rely on the TLG! It’s a remarkable tool, but far from complete, especially for Patristic texts.
The text from Maximus can be found here, in section 173. The discussion is only two hearty paragraphs, so I hope to post a translation and comments soon.