My worry is that between the time when the new data is written, and the
time when the parity block is updated (or vice versa if the parity write
gets to the disk surface first), if you have a crash, bingo, you have
instant corruption of that stripe. There isn't any way to make those two
writes happen in sync, after all.
(I *know* you know this, so am quite mystified that you're apparently
claiming that it isn't a problem. I don't see how a workaround is even
*possible*: it's why battery-backing of RAID-5 arrays is done at all...)