Styling tooltip dependent on module

[Chais]

I'm trying to change the tooltip font for a single module.
Specifically, I have a script for my custom-pacman module that outputs the list of updates as the tooltip. For that to look nice it needs to be monospaced, but I don't want all tooltips to be monospaced.

I'm not sure how I can apply the

tooltip label {
    …
}

pattern suggested in the wiki, as the label seems to be whatever the module's text is, which is not at all helpful.
I added a class to the module, but it doesn't seem to be applied to the corresponding tooltip.