OK, well there's this mod that adds some triggers:
https://vborg.vbsupport.ru/showthread.php?t=277022 . One of them is "THIS_SCRIPT value is..." which is just about the same as "is viewing the page xxx.php" except that the value doesn't have .php on the end (and sometimes it's not the same as the script name). You can find out the THIS_SCRIPT value for xxx.php by looking at the code, and near the top there should be something like:
Code:
define('THIS_SCRIPT', 'register');
or you can ask here.
ETA: I almost forgot, "register" is the one value that doesn't work because there's a condition in the navbar template that stops any notices. You can take out the condition of you want (See Note #2 in the mod description).