Post-Trent there *shouldn't* be a long interval. That is the big problem. Also, I'm not sure most people realised in early '44 the war would be over in a year.
Now, the idea of the US cardinals electing a pope on their own is just laughable. Not only aren't there enough of them, the whole process would be viewed as illegitimate by the actual curia and their pope would find it all but impossible to get any traction. The American church might find itself in the doghouse for decades after.
What would be possible is for the Allies to issue an invitation and promise of safe conduct to all cardinals, regardless of their location, to come to conclave. It is very improbable that the Axis powers would allow theirs out, but in that case the legitimacy issues are on their side (and some German cardinal archbishop or other might just choose to go anyway - I would not want to be the junior customs officer facing down Count Galen or Cardinal Faulhaber). The cardinals from all belligerent allies and neutral countries would be allowed and enabled to attend (I'm sure the Royal Navy and USN can provide transport). Some may be unwilling, but that is not an unknown phenomenon and entirely their choice. The conclave will, of course, be dominated by Italians, sand even more than usually by the curia, but that, too, is not unknown.
It would be interesting to see how various nations might be trying to influence the outcome of the election. The USA certainly has a stake in this, but I suspect the OSS would be a tad amateurish at playing the game.