Thanks, that looks promising.
I'm not pursuing it at the moment, however, because I have new info which makes the Mac log no longer important.
Although I will say, in retrospect, they sure don't put much useful stuff in the log. In this case, it appears that the iMac is configuring our device, then attempting Interrupt INs (as expected), which are being orphaned because the device is not responding. After about a thousand or so, it gives up and starts over at reset. (According to my Beagle trace.)
I think "orphaned" is how it would behave if the address were wrong (ie, let someone else answer). (But I know that isn't it....sigh).