Update, I tried decompiling the Pixel Buds app with APKLab and patching various settings, and compiling the app again. However, no matter what I tried, the patched Pixel Buds app will always start in "minimal" mode, as if the Google Services were not installed. I imagine the app somehow checks its signature before activating the extra options, so it's not possible to have those advanced options when I compile the app myself. Hence no chance to get multipoint that way.
We could still try to spoof/hide the phone model from this app directly from GrapheneOS. Is this feature available/planned to be implemented at some point?