I have worn a white tunic shirt with a cravat or a jabot but not a clerical collar and I have never seen it done ... yet! To overcome the black-on-black problem I wear coloured waistcoats. With day wear I have also worn coloured clerical shirts (blue, grey or green) but not for black-tie events. What jacket or doublet do you intend to wear? High-cut waistcoats cover most of the shirt while three-button ones tend to make a feature of it.