No reason not to do rank-based colouring with Tableau - it just needs some table calculations.
Another thing you can do to highlight the outliers when you have a very wide numerical range is to use a log scale for the colour shelf.
I wrote up some examples of using both of those techniques in a blog posting a while ago here. In that case I was using the percentile rank, but depending on the data some other ranking may be more appropriate.
Thanks for the suggestions, and the link. I'm still learning what table calculations can do, this will be one more thing to add to the list!
I second (or third) this. Having the ability to control the saturation as a variable (either continuously or discretely) would be hugely useful.