I've just had an article accepted for publication in Buddhist Studies
Review with the title 'Why Did Brahma Sahampati Ask the Buddha to
Teach?', addressing the question of the origins and contextual meaning
of this episode in the Buddha's life-story. It might provide you with
some information that's useful, and at the least is full of all the
references I could find to other scholar's work on this topic, which
might include the reference you mention you came across.
I'll send you the draft copy of the article if you're interested.