Okay, I actually got this to work. Sadly this will only work for people who already had profiles in the older launcher, but there will be a backup of the launcher_profiles.json file. Rename the new one to something else, and then rename the old one to launcher_profiles.json. Assuming you have a profile with the same name in the backup and in your new launcher profiles, you can then use MCPatcher to modify that profile as normal. Now rename the launcher_profiles files back to how they were.