I haven’t investigated much (and I will not more), but since my upgrade to Mac OS 10.8 (Mountain Lion), Safari supports safe renegociation.

Meanwhile, I had received a laconic answer from Apple to my bug report saying that they “are aware of this issue”.

Note that Safari 6.0 on Lion did not (at least on my computer, if someone could confirm)… so same browser version, different OS, the system SSL library must have been – silently – updated.

Anyway, good move finally.