"kept hush" is correct and usable in written English.
You can use it when referring to a secret or something that has been hidden from the public or not revealed. For example, "The company had been trying to keep hush about their upcoming product release."
In plans kept hush until now, 40 Wal-Mart managers are convening in Russia in April to study trucking routes, distribution points, rival store locations, shopping habits.
While Sony Ericsson was happy to show the thing off and share some of the basic spec details, they kept hush on information like the launch date and pricing.
In November, Nene kept hush on the mended relationship.
It's been kept hush-hush, discreet".
The two became lovers; Picasso was married; everything had to be kept hush-hush, nothing could stop their love, etc., etc.
The result is that a long-held belief among marketers that Super Bowl commercials should be kept hush-hush until they run is giving way to a philosophy of teasing content in advance.
In its early days, beginning in 2007, the choice of film was kept hush-hush until the projector flicked on; and trying to guess what cinematic world you were in was once part of its risky joy.
It had to be kept hush-hush, so that the 6-foot-3 Mr. Kelly could surprise the 2,400 workers at headquarters with his most outlandish Halloween outfit ever: Edna Turnblad, the bouffant mom from the musical "Hairspray".
Plot details are, of course, being kept hushed.
That wasn't the only thing kept hush-hush.
Although you hardly expect this topic to be exciting chit-chat at a cocktail party, the significance of IRAs as a powerful investment engine has been largely ignored and kept hush-hush by stockbrokers.
