I'm not trying in-band upgrade, I just want it for seeing what patches are needed. I can then download them onto the server and install them as necessary.
Why in-band upgrades are Not Supported here is weird in itself. Since it's supported on RedHat (and XenServer is a Redhat Kernel). The method it uses to apply in-band updates on Redhat is the same you use on XenServer (scp/ssh), it uses the same .BIN files to perform the updates. The only issue you run across is the disk space limitations you potentially run across on DOM0, since it's limited to 4G. If you have a lot of logs, you could see as little as 512M of free space, or less.