Couldn't you just take a lockback, depress the backspring, open the blade and then let go of the back spring... A liner lock or a frame lock is always going to have some type of snap because the lock slab has to snap into place. But on a lock back you can control the speed at which the lock engages by holding the back spring down as you open the blade I believe.
I can make my liner locks quiet the same way its just a bit more awkward to do than with a lockback. you just make sure the liner doesnt snap open until the blade is all the way out then let it go real soft like....quieter than a mouse
oops guess that was already adressed by firebert....I should read the whole thread b/f I respond