if you contacted the manufacturer of the bike and asked them if the bike could be backpedaled in the big-big combination without jamming the rear dérailleur, they would, IMO, say, yes. that's enough, in my book, to make the question legitimate.

would i make a habit of doing it? probably not. would i backpedal in the big-big combo on rare occasions? (BTW, if you watch an excessive amount of pro racing , like i do, you would notice that this is probably more prevalent on the pro tour than in amateur club riding) probably so. would i take a look at the alignment of the drivetrain? yes, i would.