I managed to find a USB-to-serial adapter and tried some things with that. My Windows machine won't recognize it, and on my mac I can't figure out what tty port it's on. I did however dig out a Raspberry Pi where it seem to work.
Unfortunately it didn't help, because the linux-version of the pycom updater just keeps saying "Something failed trying to connect to the device." and then gives no further info.
I also tried modifying the baud rate in the updater.py script as some thread suggested, but no luck.