Well, let's see what I can do for you, me lad.
You take the plate - apply a thin coat of 'washing up liquid' or 'Salal (should that be 'salaD') Oil'.
Do you water down the washing up liquid? Are there any other general household 'liquids' one can use?
You can generally use any kind of detergent. Some people prefer to water the washing liquid down, but I don't. Chances are that the GS you'll be pressing into the mould later on will stick to the mould if it's TOO watered down. Just before of making it foam.
Press the plate into your greenstuff.
Do you leave it there, until the GS hardens, or prise it out straight away?
I was worried about disturbing the mould?
You can do both, but I'd recommend leaving it in there for at least one or two hours, till the GS STARTS to harden out.
Once you have your hardened mould, I assume you then apply a thin coat of the oil or washing up liquid to the mould, and then press in your fresh greenstuff?
That's just it.
Again do you leave the new GS to harden in the mould, the oil coat stopping it sticking to the mould?
That depends on what you actually plan to do with it. If you want to use it in exactly the same shape and kind it's originally used, leave it till it's hard. However, if you want e.g. to aplly a symbol to a curved surface (like horse barding like I did with my Reiksguard etc.), better just leave it in there for about an hour, then carefully take it out (it's not hardened out yet and one could still screw it up badly now) and apply it to the corresponding surface.
The "other bitz" I was referring to... well, basically, you can do this with every part that only has one modelled side (or only NEEDS one). I did Thigh plates this way, but also pistol-holsters, many symbols from various sprues and models, bucklers, I once even tried double-handed swords that way, they weren't exactly bad, but it showed the limitation of this method, because it was more or less luck to have BOTH sides look well. That's why I only recomend it for parts with only one modelled side.
As for the legal stuff: I think Kaz is perfectly right here. If you officials get the legal stuff worked out and decide that it's ok, just drop me a line and I will see what I can do about an article.