I seem to be unable to transfer my character from Ultima IV to Ultima V. I was able to track down the PARTY.SAV file, but after copying it to the appropriate location for ULTIMA V I get a message: "Error: Your Ultima IV game contains bad data. Unable to continue transfer."

This is on a Mac, btw, so the paths are different than on Windows. I copied PARTY.SAV from ~/Library/Application Support/Boxer/Gamebox States/com.gog.ultima-4/Current.boxerstate/PARTY.SAV to the com.gog.ultima-5/Current.boxerstate directory.

Has anyone gotten this to work on a Mac?

I had the same error and figured out a solution, though the way I transferred my character was a bit different than yours.

You have to be familiar with how OS X Bundles work to get this working. Basically your Dosbox/Boxer VM needs to mount a new "hard disk" drive containing your save. The only way I could get that to work was to follow the instructions here:

boxerapp.com/guide/advanced

Basically copy your PARTY.SAV file to a directory inside the Ultima 5 Application bundle. To do this, right click on the Ultima 5 application via Finder and select "Show Package Contents"

Navigate to Contents > Resources > Ultima 5.boxer

Right click that and select "Show Package Contents" again. You'll see a "C ULTIMA5.harddisk" folder plus some other files like Game Info.plist, etc. Create a folder "A ULTIMA4.harddisk" on the same level as the "C ULTIMA5.harddisk" folder. Then drag your PARTY.SAV into that new folder.

Run Ultima 5 and transfer your character. Things should work.

Note, instead of making a ".harddisk" folder I initially tried to do this with a ".floppy" folder. I got the same error you did. Only by making this a "harddisk" worked. Again check out that link above for specifics.