I think to dip or not to dip is a great question.
Some related observations:
Most public swimming pools request/require that you shower before entering their pool.
A pediatrician in our community solved a vexing problem a couple summers back when he observed that all the kids he treated for a nasty GI bug had visited the same wading pool. Turned out a carrier for a bug had an "accident" in the pool. The wading pool's treatment and filtration system wasn't able to remove/kill the pathogen, resulting in many cases of a nasty but short-termed illness.
Portland, Oregon has several open air drinking water reservoirs. When a man was observed urinating in one on security camera, the reservoir was taken off line, drained, and sanitized. Here's a link to one report: http://www.nwcn.com/news/Man-urinates-i ... 07264.html
These things said, it has always puzzled me that bathing in the drinking water supply doesn't bother people when they step onto the trail. I remember reading about the dark ages, India, and the third world that many public health issues were linked to contamination of drinking water by bathing and waste disposal. Somehow we are insulated from the consequences of our actions around water by??
And I have to confess I have dipped. Guilty, Guilty, Guilty. So no high horse here. B
I think your idea of a prewash is a good one. I carry a light weight plastic bowl and small microfiber towel for such purposes. Not that I use them much. For example, when I did the JMT in September a few years back, I dipped. But not often. As the cook, by the time I cooked the evening meal and reorganized the food for the following day's travel, it would be getting dark and cold, and it was a whole lot easier to crawl into the sack than it was to get soaking wet.
Have a great time on the JMT. I did.