I have this same problem. I've had it for quite some time now. But I'm not running OSX. A few pages back you mentioned a few people using "use_pickle" for embeded versions of linux. (I use OpenELEC). I've tried that and it made no difference.
It can happen day or night. It can happen when downloads are paused or any time during the download. Now since it's been happening for a couple months for me I've noticed it rarely happens during small downloads. Most of the time's it has crashed it's with a file over 1gb. The bigger the download the higher the chance to crash it seems.
The thing is I never had any crash happen when I was using OpenELEC version 1.. it wasn't till I started using nightly's (and now current RC's).. that it started to happen. So most likely a change in version for SAB or Python change triggered it.
If I can think of anything to help you track it down I'll post. But thank you for putting as much effort into this as you have. It seems to affect a very small amount of us.