I have picked up a few things in reading, you may or may not already know this stuff but throw it out for comments.
A restrictor is basically designed to be a critical flow nozzle. When flow through the hole hits the local speed of sound the flow cannot go any faster. This leads to some interesting things.
One way to increase flow is, as you mention to increase the density of the air. Higher density, same velocity equals more flow. The bad news is the local speed of sound is most dependent on air temperature, so as you cool the intake air, the critical flow speed drops --- that means you don't get quite all the increase you would expect from the change in density. Your mass flow will increase at the square root of the absolute temperature change and not directly with the air density.
I haven't worked out the numbers but you should still get some gain. You will want the most cooling with the least evaporated liquid to dilute the air charge so straight water with a small amount of alcohol might be best, depending if the rules allow use of alcohol. Inject far enough upstream to give you some time for cooling to take place.
The other interesting thing is that max mass flow through a critical flow nozzle does not change with increased suction on the discharge side of the nozzle (restrictor plate), sonic choke velocity is still sonic choke velocity. BUT -- total mass flow will increase if you increase the pressure on the inlet side of the orfice. Even though the air flow velocity is stuck at the local speed of sound, you still can increase flow by having the maximum air density possible on the inlet side. That means careful attention to pressure drop on the intake prior to the restrictor and any ram air configuration you can set up will help mass flow.
Third, flow through a critical flow orfice composed of a sharp edged hole in a thin plate (as in NASCAR restrictor plates) is very sensitive to the entry conditons of the hole. Even small buildups of dirt that tends to round the edges of the holes inlet side can increase flow. If its a restrictor that has some manditory length like the inlet restrictors used in SCCA for turbochargers thats not much help. There is not much you can do, but give the best possible smooth entry to the opening and make sure the flow is as uniform as possible.
If your rules allow you to modify your intake plenum size, I understand that a large plenum will make the engine see a larger restrictor size as each intake stroke has more air to draw from the plenum with less pressure drop during peak flow. This slightly delays sonic choke as the average flow through the restrictor is more uniform.
The guys in NASCAR have on occasion been caught "accidently" creating vacuum leaks down stream of the restrictor ------ Like accidently putting gaskets on the carburators that would leak after they got hot ----
Not that I would recommed you "bend the rules" but ------
That is all the info I have on the subject.
Larry