This isn't definitive, scientific proof for end-game ACC cap considering the differences, but it's good enough for me to believe the ACC cap needed for each position is indeed different:

As level 45 WAR, with 340 ACC, fighting a level 50 training dummy

Fighting facing the front: 465 swings of auto attack. 427 hits, 38 misses.
Fighting facing the flank/side: 237 swings of auto attack. 232 hits, 5 misses.
Fighting facing the rear: 300 swings of auto attack. 300 hits, 0 misses.

Lots of variables could make a difference, like the fact I'm AAing only, the test isn't exactly the longest to be consistent with the statistics for sure, and it's just a level 50 training dummy, but it seems extremely likely there is in fact a different ACC requirement for each side. Also of note, my BRD friend came by, stripped naked so he only had 12 more ACC than me, and missed about 2% of his attacks to the front, but none from the rear.