Universal's upcoming film "The Pirates of Penzance"--starring pop singer turned budding actress Linda Ronstadt--is still seven months away from release, but already it's stirring up a controversy. It started last week when one report quoted MCA president Sidney Sheinberg as saying he's considering a plan that would premier the film, on Christmas day only, on pay television stations in eight major markets; it would receive a standard nationwide opening the following day. The National Association of Theater Owners (NATO), who have some stake in getting people out of their living rooms, took that announcement to mean that Universal had already made up its mind, and NATO president Richard Orear screamed that his organization deplored the plan, which he said constitutes a "serious threat" to the entire industry. No sooner had Orear spoken his peace than Motion Picture Association of America president Jack Valenti jumped into the fray; he too was acting on the assumption that Universal had firm plans, although he admitted that since he hadn't spoken to Universal he was taking Orear's word for it. At any rate, it won't hurt theatrical exhibition to have a single broadcast on subscription TV, Valenti insisted. One can only guess at how heated the arguments will get if Universal ever really announces a full-fledged plan . . .