I ended up not using multiboot, and running other stuff with VMWare Fusion, but kept running out of space in the MacOS partition, as many things like to be there, and are tricky to relocate. But I had an unused 10GB partition adjacent to it. Any chance I could combine them and expand the HFS+ filesystem to take advantage of the additional space. At first, I started looking for an external drive to stage everything on while I repartition the drive and built new filesystems, but then I remembered I'm using a Mac.
So I went to Disk Utility, combined the partitions, and dragged the filesystem to fill the larger space.
And it worked. While I was using the machine. I magically had 10GB more space, and I didn't have to shut down any programs, log out, reboot, or anything like that. I actually did this while I was chatting with people, and the only bobble was the network connectivity bounced, so I vanished and reappeared a few seconds later.
Now THAT impresses me, all the more so, given my (now) intimate familiarity with the complexity of changing filesystems and partitions. Good on ya, Apple!