My thanks to everyone who responded to my question.
Dr. Bhattacharya, I did get your reply yesterday, and I suppose so
did the rest of the list too - maybe for some reason it ended up in
a spam filter for you?
Judit, special thanks for your detailed response. (Is your article
on the net somewhere public? Or would it be too cheeky to ask for an
e-copy?)
I feel that speculation about the direction of borrowing will have
to remain just that, speculation - but it's nevertheless
interesting. In my particular case, the
Mudrārākṣasa, I'm sure there are at least three stanzas in it that
are borrowed
"public domain"
subhāṣitas, and two more including the one with
Śeṣa that may well be such or may have gone the opposite way, going
from the play to common subhāṣitas.