MarkT:
Leave isolate=0 and stop worrying? Set it to a stupidly large value and see what happens?
Ah yes, the stupidly large value, great idea, now why didn't I think of that! XD
I tried a 100 mil isolate. Not surprisingly, what happens is the ground plane falls apart into multiple polygons, breaking ground connections in several places. Airwires appear that make this obvious. But the risk may be that with a not-so-stupid-but-still-too-large value, the same thing happens and the airwires are so small they don't get noticed. This is one of my pet peeves with Eagle, very short airwires can be hard to see. But there's no excuse for not knowing they're there, because the ratsnest command indicates the number of airwires on the status bar.
Still confused on the language in the help item, "small gaps may result ... which may lead to problems during manufacturing" makes it sound to me like the gaps are undetectable or nearly so. But as long as an airwire gets created, even if it's hard to see, ratsnest lets us know it's there.
Thanks for the suggestion!