I know it may possibly not seem wholly in the spirit of the LookAndFeel class, but the setColour/findColour function is an excellent way of quickly adding colour definitions to an app, without having to subclass/cast LookAndFeel all the time. You can just add a colour at the start of a program, and refer to it where it’s needed.
Now, I think it would be very handy if a similar thing could be put in for general appearance values. E.g. the width for label text of some custom component being used in your application. It could just be LookAndFeel::setLookValue(int id, float value) and float LookAndFeel::findLookValue(int id). I suppose you may not think it’s what LookAndFeel should be used for, but when prototyping it could save a LOT of time in neat consistent interface design.