Hi Ken,
by PP I did mean post processing, but I was being lazy in that I meant processing the image in photoshop post capture.
I'm familiar with using selections in photoshop to only manipulate certain areas of the image, but unless you are able to capture the detail of the highlights in camera, then no amount of selective photoshop work later will bring them back.
Obviously you can bring in some sky from a separate image to replace lost detail, or use a tripod to take a series of separate RAW exposures and then blend them together in photoshop later, capturing your highlights with a faster shutter, and your foreground/flowing water with a longer exposure. I was just drawing attention to the fact that filters can allow you to achieve this in one step.
Without using filters or multiple exposures then your final image will either be more of a compromise, or a real photoshop hatchet job.
Here's one example:
2 seconds, f22, ISO 100 - Sky and tree detail lost in blown highlights, foreground rocks under exposed
4 seconds, f22, ISO 100 - Grad ND8 and Grad Blue filters combined reduce light from sky, reducing blown highlights, increased foreground detail.

I know which one I'd rather start work on in Photoshop