Ok, the trick was to find the ID in reverse, as explained here: https://www.soundonsound.com/sound-advice/resolving-vst-plug-conflicts
So if you want to duplicate your AU/VST (I only tested VST, but AU should work the same I suppose), follow this:
-find the ID of your plug-in, I used MuLab (load the plugin, right click and "Show info"). Also get the name of the plugin in MuLab
-Go to your Plugins folder, duplicate the plugin, rename it, and right-click and "Show Package Contents"
-Go to Contents/MacOS
-Open the binary with a hex editor (like HexFiend)
-Search the string as text, but backwards (if, for example, your id is ZjQ4, look for 4QjZ)
-"Replace All" with 4 letters at random, and save the file
-Rescan the plugins in your DAW and you should be good! (I personally needed to trash the preference files of my DAW because the old ID persisted)
You can also change the plugin-name using the hex editor, if you want to differentiate between the original plug-in and the duplicate. You can find the original name using MuLab.
Edit: Actually you are required to change the name of the plugin in the binary, or else it won't load (at least for me). Look for two strings before the first ID occurence.