[takoi]

Hmm. I've tried just starting it and letting it stand, but nothing appears in the log after "Unloading: libImageLib-arm.so" until the automatic shutdown triggers, and then the exact same hanging behaviour occurs (http://xbmclogs.com/show.php?id=191234)

The network share is connected to through ethernet, running on a fast server PC, and I've never had any noticeable delay when browsing it or playing video from it...

Is there any way to tell by watching the log or anything when the watchdog finishes scanning?

After line 476 in your log, it should print success ("watching ...") or failure ("failed to watch.." or "does not exist"). After that, it has finished initializing.
It first check if the path exist. If there's a broken connection this can hang up to a minute. Then it will check every file and folder in the path. If it's extremely large, this can take a while, but 5 minutes seems unreasonably high to me. Nevertheless, try leaving it on for a longer time to see if it finally does succeed.