Hello G.,
I think that is not due to limitation of PlantUML, but more (for png or svg output) to limitation of tooltip itself: managed by title and/or xlink:title in cmap (with <area> tag) and xml link (with <a> tag).
A first step will be perhaps to add Unicode and/or entities management on tooltip: I will ask this kind of wanted features.
If that can help,
Regards,
Th.