Hrm... maybe I was a bit overzealous?
Space Shadow doesn't appear to flash the whole screen, but only flashes the target. Which I guess would work as long as nothing else on the screen is moving.
As for holding the trigger, holding at full pull wouldn't work due to the way the zapper works. If you have one, pull it out and follow along with the below:
The trigger has 3 states:
A) Not pulled
C) "full pull"
Pull the trigger very slowly, and you'll feel/hear it transition between these states. The A-B transition is very subtle, and has a very soft "click" that most people don't notice.
The B-C transition is the extremely loud and noticable "CLANG" that everyone thinks of when they think of the zapper trigger.
The problem is, even though the trigger has 3 states, the zapper only returns 2 to the NES. It gives a '1' if the trigger is in state B, but gives a '0' if the trigger is in state A or
C. So there is no way for the game to know whether or not the trigger is being held down fully, or is not being held at all. Most games look at the 1->0 transition ("CLANG") to see when the trigger as pulled.
keep the trigger in state B and do a rapid-fire that way, but it would be awkward, as you'd have to be holding the trigger in a somewhat narrow spot between the click and CLANG. So theoretically possible I suppose, but it'd be weird.