Adams Arms has solved this problem. 3 position gas block: Normal, Suppressed, Off.

Isn't the sound of the action opening and slamming shut already louder than any venting gasses?

From memory it's louder than my MP5 clone, but quieter than my Uzi.  Mine may be better than normal as I am using a Hahn Precision .45 buffer, which seems to do a good job of helping with breech opening timing.

CZ Scorpion :)

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Ding ding, we have a winner.

Just hold your support hand tightly over the ejection port.

What could go wrong?

move the gas block further up the barrel. venting much more forward means less gas to bleed if it's too much. also, using more of the lower power gas than using a little of the high power is easier on the bolt and will result in less force = less noise. the action should then vent downward, away from the shooter's ears or other persons. the longer barrel will also burn more powder, which makes it easier to suppress at the muzzle. add an adjustable piston to comfortably cycle the action. the longer barrel would keep it lethal at longer ranges too.

but alas, that would require a complete redesign of the rifle into some kind of downward ejecting rifle of some sort. despite this, i firmly believe that adhering to the antiques of old is still the right path.

If we're talking piston systems but is the OP referring to a DI or piston system.  2 different animals.  I have two sig piston guns the 5.56 is top heavy and way over gassed, the 716 is a really heavy beast but they at least got the gas right even when shooting suppressed.  

A DI system that vented the gas forward of the receiver or bled it off at the gas block  would be ideal.  The govnah was a great idea unfortunately he's gone.

Superlative Arms makes an adjustable gas block that bleeds a certain amount of gas at the gas block.  I don't know if that is quieter than an SLR block, Govnah or not.