++ (x3) Thanks for the effort you put in to investigate this. It's appreciated.
Yes, 64kB is roughly the number I seem to remember seeing in documentation somewhere on this site; unfortunately, I can't locate this and don't remember the exact number. Also, I'm pretty certain I've never attempted to post anything that large.
It looks like implementing this request would involve a massive amount of work and, as such, probably won't happen. That's fine: it's just a feature, not a bug fix, and the situation that I described only occurs rarely.