Rowan, you asked:

I've always used the construction "A propos your post...." by virtue of the French origin but usually omitting the accent over the "a". But I find I'm in a diminishing minority, outnumbered by those who insert "of" immediately after it. Why do think that might be?

Why people insert "of"?   Damfino.

I would never think to put an "of" in there, and I haven't been particularly aware of that usage.   On a nonacademic basis, I think of the use of "of" in that connection as a mistake.

Dave Oesterreich