Following the users@ thread entitled "Custom CSS for Feedback message is broken in 1.5", please find attached a patch that provides #getListCSSClass (applied onto <li /> elements) and #getLabelCSSClass (applied onto the underneath <span /> element).
Both methods return #getCSSClass by default for backward compatibility.
Please note that #getCSSClass has been marked as deprecated (and to be changed to private in next version) to prevent confusion. (imagine the user overrides all 3 methods for instance, its #getCSSClass will be ineffective).
The patch has been done based on wicket-1.5.x branch (github). I did not see a relevant dev branch for 1.5.9 specifically.