This depends on what effect you are looking for.
If the menu item just contains some text that changes when holding down the Option key, then this is fairly easy by adding two menu item, setting the alternate option on the second one and assigning it a keyEquivalentModifierMask of NSEventModifierFlagOption.
If you need the custom view for something more complex than just text, then you would need to install an event monitor. (+[NSEvent addLocalMonitorForEventsMatchingMask:handler:], let me know if that is the case and I can provide some sample code.)