I am sure that the output bearing is lubed by splash (only), and that the splash occurs only when the input shaft and cluster gear spin. I also know that the warning about flat towing without removing the d-shaft is general wisdom for any RWD car with a driveshaft (for standard and automatic trannies). The rule of thumb I always heard was 25 miles. Perhaps Volvo transmissions are more robust.

I've personally never had a failure -- because I've always removed the driveshaft for anything over 10 miles. (The one time I towed with a dolly, the front wheels were on the ground.)

"How much lube does a bearing need if it's not doing any work?"

Keep in mind that the load on the bearing does not depend on the load on the car -- it only spins and supports the radial load, which is the weight of the driveshaft and related stuff. So that bearing is doing the same "work" whether you're towing the car at 80 or your screaming down the road at full turbo boost. The bearing load isn't related to engine load.
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Don Foster (near Cape Cod, MA)