The most difficult issue here is to find the right syntax...
Maybe we could indeed use a new syntax.
What about introducing 'style pseudo-stereotype' with three guillemets <<< >>>.
This would apply the style to the class without adding any stereotypes:
@startuml
<style>
classDiagram {
.mystyle {
FontStyle Bold
}
}
</style>
class Alice <<<mystyle>>>
@enduml
Would that makes sense ?
What people think about it ?