Answering the original question, no, American manufacturers aren't incapable of fielding LMP1s. But why would they want to? The only time LMP1 machinery will even race in America is during the FIA WEC event at CoTA this September. And even then there have been rumors about the feasibility of that event going forward, and some people think it may soon only be a TUSC thing. Then you're left with the Montreal option, which is regional but not "American."
It just isn't logical, I'm afraid. There's no need to waste money on a global championship when they don't even have any interest in it.
But one can hope... I do hope we see a Ford LMP1 one of these days, even if it's only a render!
