Ad someone pointed out on pg 1, steam does a great job of cleaning as evidenced by a bad head gasket leak.. No, it won't do a whole lot on the backside of the intake valve which is too cold and not under any significant pressure changes to have anything happen.
I think one of the main reasons water/steam cleans the combustion chamber so well is that if the carbon is wetted with liquid water on the compression stroke, the increase in pressure will drive the water into any porous parts of it, as ignition happens, even more so, then it gets really hot and the pressure decreases, so that water turns to steam, since this happens very rapidly and the pores are small, it breaks the carbon apart.. These conditions are not present on the backsides of the valves though.
How much water is too much? Well.. I'll take a Cummins 5.9L diesel since I know the exact dimensions.. and because every cylinder has pretty much a 1L swept volume.. I know from when I built my engine that the piston dish is 45cc, and there's about another 10cc of volume between the piston and head.. so 55cc of total minimum volume. Any introduction of liquid water will evidently increase the compression ratio, however, true hydrolock will only happen at about 54.9cc. That doesn't mean that you don't get excessively high pressures before then.
Knowing that this engine can handle boost pressures of 45psi (4x atmospheric), I think I'd be safe to say I could reduce volume at TDC by 50% assuming it's NOT at boost and have a decent safety margin yet (I'd have to do the adiabatic compression math to find the exact amount). Anyhow, I would say that an engine can actually ingest a considerable amount of water when it's running before you get excessive pressure.. However, POURING it in is far too unmetered.. a little bump of your elbow would make for a bad day, however, using a nozzle and needle valve would give you a steady, metered flow that you could hear how the engine responds and modify the delivery accordingly..
So at 2000 RPM, that's 1000 intakes strokes and if I allow 50% of minimum volume to be water, that's 1000x ~25cc = 2.5 liters per minute.
Here's a another company that does water/alcohol injection, and they have a calculator on their website to choose the right nozzle sizes.. I've been very tempted but haven't bought it yet
http://www.alcohol-injection.com/en/