I've defined a number of custom publish types (namely "PortfolioPage" and "PortfolioGallery").

All of them utilizes a custom taxonomy known as "Artist".

Can there be in whatever way to limit the finish-user to go in For The Most Part ONE artist per PortfolioPage / PortfolioGallery publish?

You can make the custom taxonomy to make use of radio buttons instead of checkboxes, take a look here: http://wordpress.stackexchange.com/questions/7456/altering-the-appearance-of-custom-taxonomy-inputs