I confirm that when AA says they upgrade internationally using miles from any published fare, they mean it. 2 years ago they had a very low $300 or so fare to LHR over the winter (including taxes & fees!) and I had no trouble upgrading with miles.

UA now processes int'l upgrades on a one-way basis: 10K each-way from Y or B, 20K each way for lower fares, although (as noted above) they won't let you do it from the super-low fares.