Yes, Ringway would be ideal for pre-clearance!
Well costs on the US side dictate that you are only going to get one facility rather than ones in multiple terminals; that is if the UK law ever changes to allow it to happen. And that's why nothing seems to be happening at present although the Government have been discussing it again recently.
In practical terms;
Do you put all the US flights into one terminal? And you therefore have some airlines with split terminal operations?
Or have some airlines use it and some not? I guess some airlines would be a bit hacked off by that
And how does all this square with making good use of all the terminal capacity?
Maybe all well and good if the rumoured single terminal happens, but before that I would suggest it could be a bit of a dog's dinner.