For an abomination, behold! The roundel of the Royal American Air Force:
View attachment 177521
EDIT: as a note, it was a simple recoloring of URUK's roundel
Ugh! That
would be abominable!
Also implausible; the USA gets its star motif from independent republicanism; a Royalist BNA would not be using them.
Well of course the Australians and NZ use them OTL--but I suspect that was a rather defiant inspiration by the American model, suitably modified. Those Aussie star flags started out I believe as the banners of insurrections against British rule. (Bearing in mind, many Australians were convicts, and others were people who hadn't had good luck with the prevailing system in Britain.)
However it's a perfectly good alternate roundel for the USA joining a British-led alliance, as when we did join the Allied side in WWI.
I happen to think the US roundel that prevailed from after WWI to just after WWII started--the round blue field, the imposed white star with the red dot just touching the interior corners of the star--was really really cool. It looks great on Navy rigid airships in particular!
We abandoned in in favor of the two versions of the bar-winged white star on blue field (all blue-white wings for the Navy, with a red stripe for Air Force) because apparently at a distance the red dot was most visible and looked too much like the Japanese insignia.
Too bad though. The old roundel still survives in some insignia where it has golden stylized bird wings.
But I'm not sure we already had it during WWI itself. I think possibly some US planes had pretty much your roundel!