I've never tried it, but I am sure some others here have. Since "instant" grits, polenta, oats, and so forth are pre-cooked and should be gelatinized already, you may be able to add it directly to your mash, but I'd probably go ahead and do a regular cereal mash with some two-row anyway. I can't say how it would perform compared to non-instant polenta.
Reality is a crutch for people who can't cope with drugs. -Lily Tomlin