If it is just changing a class you could add a check like if($_zp_current_zenpage_page->getTitlelink() == "") { .... } and let it print it.
You could also modify the theme to use for example the custom field or a codeblock to store a class or id name and just check for it.
The multiple_layouts plugin is best suited if you do really different things like different columns or something.
Thanks for your quick answer.
I guess the custom field or the codeblock with a theme modification will be me best option: whenever I want to have another page layout, I'll just have to add the corresponding CSS rules.
But I'll have to investigate a bit, as I have yet to use a custom field or a codeblock.