Technically there are ways of doing a mass join using multiple hosts, but it's not the simplest method, and correctly identifying a mass join that uses multiple hosts vs. a lot of people joining at once (such as after a net split) isn't simple either.
Generally speaking, it's unlikely that you'll actually encounter a situation where one person tries to do a mass join using multiple hosts.
I've been an IRCop on a couple of networks for a couple of years now, and, to the best of my recollection, I've only seen it happen once.
Not to be disagreeable, but it happens with some frequency on some of the larger channels on undernet. Depending on who's mad at what channel you can see them anywhere from once a month to nearly daily.
The easiest way to handle mass joins like that is to set the channel to +m if the joins continue for more than 3-4 seconds. Netsplits all come back at once, so it is possible to see the difference, though it does take a fine touch at times.
