What do you imagine the net effect of having the valve open after the projectile leaves the muzzle to have on overall system efficiency?
IMO, you're going to waste a lot more air with little gain.
Best to have the valve close when the projectile is about half way down the barrel, and deal with the hammer bounce.
Solution put forth by Mr John Bowkett....you MAY have heard of him.

http://www.sumobrain.com/patents/wipo/A ... 3912A1.pdf
Harper sling shot hammer....Daystate
http://www.network54.com/Forum/79537/th ... %3B+design
Also do some searching for "bstaley o-ring mod" which limits valve stem travel and total dwell. A nifty side effect of reducing valve stem travel, is that the valve stem imparts less energy to the striker when the stem closes.

Another fellow developed a simple brake mechanism for the Crosman airguns. Searching for hammer debounce device will turn up a goodly amount of information.
The Hatsan AT44 has a mechanical "catch" for the striker...preventing it from hitting the valve more than once. A trigger/sear (disconnector) mechanism as used in semi auto firearms would also catch the striker....