Even just moving it is fine, because I don't expect my parts to do goofy things as I move them, even if it's between grid domains; but at least snapping the angle when I hit space, that would be nice.
I suppose the lesson is, nobody uses it enough to warrant those features. I've never used custom grids, personally.
There's also the slippery slope that, since the polar grid is a thing which exists, you'd have to provide routing with straight (radial) and arc segments. (Presumably, limited to 90 degree corner mode, as there's no Gerber primitive for a 45 degree diagonal polar segment -- a spiral.) So, there would be an awful lot of work to bring this rarely-used feature up to a level of consistency that would make sense.
And the autorouter has to be aware of it, and...
Tim