Plain white rice starch granules are small and dense and take a higher heat to gelatinize from what I understand, although I couldn't find a temp in a quick online search. Not sure you'd get a good gelatinization at mash temps even if you milled it first.
By the way, is rice much different than just using sugar? I did see that it is 20% amylose, 80% amylopectin, so I suppose there would be some dextrins left after mashing, that you wouldn't see in table sugar. Is it unique in what it brings to a mash as an adjunct? Or is it primarily used in commercial brewing because of cost?