While deep in the bowels of S2, trying to track down something unrelated, I stumbled across the allow_others attribute for properties, which allows custom values to be saved for properties which usually show up in a dropdown.

Possibly helpful for things like oh, date formats or margins or whatever.

There's no frontend for it, but it would be simple enough to make it so that if you have a custom value (say one entered via the layer editor), it would show up in the dropdown (so it wouldn't be wiped when you use the customization wizard). Otherwise, just show the default set of values (as before).

Another possible interface would be to have a "custom..." option which would show a hidden text box using JS, but that's a bit more work and probably not worth it unless we had a specific use case in mind.