Well, finally got an opportunity to do some flushing. Don't know if this was the ideal set-up, but this is how I did it:
This was to flush the engine half of the system. When I first flushed, I left the pressure gauge off, and just let some water come out of that opening. After a minute or two, turned off the water and put the pressure gauge where you see it here. (It was just this little cheapo made in China thing---all I could find locally. Got some rubber stoppers, drilled a hole through one and screwed the gauge into it.)
To flush the radiator part of the circuit, I put the hose (with the plumber attachment) into the big hose that normally would attach where the t'stat is.
Ran MANY gallons of water through both circuits and you know what? I got squat. I sieved every ounce of water and saw, essentially, nothing. (Did see two tiny pieces of something---much smaller than the head of a match---that might have been miniscule pieces of impeller blade. It was hard to tell, they were so small.) Funny thing is, I never got the pressure gauge to register anything above zero---pointer seemed pegged there, never moving once.
Don't really remember much fluid dynamics, but I wondered if, with the water moving past the gauge with a certain amount of speed, it created a relative low-pressure area. Like air moving over the top of a wing, or like when someone's smoking in a car and opens the window---the smoke rushes out towards the low pressure zone (ie outside). Dunno. I crudely tested the gauge at one point: held the hose directly against the opening in the stopper and turned it on. Lots of leakage, but I could get the gauge up to about 30-35 psi doing this.
So I don't know. Maybe I wimped out on how much water pressure I was putting through? Seemed like it was a fair amount, but with the gauge telling me NOTHING, I wasn't sure how far I should push it.
Emailed a guy that I've worked with before, works at this big shop that specializes in pcars. I've seen EVERYTHING imaginable there, Porsche-wise, including many track vehicles...they always have at least 6-8 Porsches at any given time. Anyway, he emailed back about their experiences with water pumps. Some excerpts:
We have done probobly 25-30 water pumps on Boxsters and a few 996's. I have yet to see one come out with the impellers intact, the bearings fail and the the impeller hits the block and breaks
The very first one we did (had to be 6 years ago) the impeller was gone, all of it!. We pulled the radiators, all the hoses, back flushed the system. Never found any of it! Put it all back together and sent it down the road, he still has the car with no problems.
We dont even bother flushing them now, it is never an issue.
I would...say you have nothing to worry about.
I've had several other knowledgeable sources give me the same advice.
So, while it's clear that some people have seen problems with this (JR), it seems to be relatively rare. In a perfect world I'd have a 3 car garage with a lift, two reliable back-up vehicles (instead of one---a '96 Camry with 195k miles), and lots of time on my hands. As it is, I really don't see me tearin' into this, looking in every nook and cranny trying to find 1/4 gram of plastic. Sooooo, unless I come up with any other ideas, I'm puttin' it back together and goin' with it. Mine will be the test case car, eh? Life...it's a crap shoot, right? You get lucky or you don't. Time will tell....