Can I clamp the turbo coolant hose?

Are you losing coolant? I don't think clamping the hose will work as you would still have coolant inside the turbo. I also would be surprised if the problem was the turbo. If it was, it should leak all of the time.

Is the car hard to start? Does it seem like a misfire when it does start?

The coolant can't mix into the exhaust. It goes through a cavity in the bearing housing, but unlike the engine oil, does not contact the shaft or seals.

If the housing cracked it would, but it would happen all of the time, not just on startup.

The white smoke happens worst at startup, and only once in every ten starts, or thereabouts. The car is also burning oil, which is constant, and distinctly different from the billowing white. And oil is also leaking (slowly) from a newly replaced valve cover gasket, at the back of the engine.

Today we found two clogged pcv tubes. I recently replaced several parts in the pcv system that were visibly bad, but it never occurred to me that the steel tubes could be clogged solid. The vacuum booster was also clogged.

After cleaning all those things out, there is still a lot of air coming from the oil filler cap, more than I've ever experienced on any other car. Tomorrow I'm going to put a gauge on that cap and see what's the actual pressure in the crankcase. Can anyone tell me what's a normal pressure? Also, what's the normal vacuum pressure at the manifold? I'm going to check that also.

Oh, and to answer your questions PZ, yes I am losing coolant, no the car is not hard to start, no it does not feel like any misfiring is going on.

There are also whitish brown deposits on the plugs, caked up and pretty hard rather than gunky like oil might be.

Then I would suspect the turbo housing is cracked. Usually a head gasket, cracked head or cracked block will steam clean the plug and cause a misfire on startup.
I would pull the cat and pressure test the system to look for coolant leaks from the exhaust side. Also check the turbo for play in the shaft as that may cause the excess oil use.

How many miles on the engine? If miles are very high, there may be more blow by past the rings than the PCV can keep up with. I've never measured crankcase pressure, but vacuum should be close to 20.

Thanks PZ. A cylinder leak test showed all cylinders between 17 and 25%, which is normal I think. Well within the green on the tester.

I used a manometer to test my crankcase pressure and it showed about 1"wc of positive pressure. If the pressure is supposed to be about 20psi negative (did you mean wc or psi?), then I've got too much pressure and the PCV system is still not working.

The car has about 140k miles.

Pull the catalytic converter to check for coolant loss? Never heard of that . . . but I'm a shadetree after all. Please explain.

Manifold vacuum should be 20 (inches of mercury is the measurement). Not sure about the crankcase pressure.

Since the leakdown was good, you can rule out the headgasket. That leaves a cracked turbo housing and a cracked head as possible causes since there is no external leak.

If you pull the cat and pressure test the system, you may see the coolant leaking out of the turbo.