Excuse my multiple replies.
I realized that I don't need the password's last occurrence; I just need an occurrence of it that is accompanied by an indication of the user's identity, such as their e-mail. If I have those two pieces of data, for presence of which in $_POST I may test anywhere, even in index.php, I may do the verification with respect to the old database before authentication proper even starts.
If someone knows the proper place to do this (in particular as to avoid overwriting at Presta updates; perhaps I'll place the check in code auto-prepended through .htaccess if it would persist), please share, but I'll leave the above for anyone facing the same conundrum.