Sub-numbering and keywords

I’m trying to tweak the comic script template to auto-number the dialog balloons on a page (in line with the Fred Van Lente style). This is the first time I’ve tried to fiddle with placeholders, but reading the help document leads me to think I should be able to stick this in front of the character names: <$sn:dialog>. I use the keyword because I don’t want to lose the sub-numbering for individual panels.

Sadly, it doesn’t appear to be processed by the compiler. It looks like I can use <$n:dialog> with a <$rst_dialog> before each new page, but the $sn approach feels cleaner. Is there a reason it doesn’t work? Am I missing something?