Did anything come of this? I'm attempting to solve the exact same problem as you (moving third party library that is only available for Intel into an XPC service) and don't have immediate access to an M1 machine to test. I've compiling and running my main target as Universal and XPC service as Intel just fine so far.

This perfectly addresses my concern. You are correct this is a nested appex, and that appex has its own embedded XPC service. I wanted to make sure that malicious software could not intercept data between the appex and the service. The data in this case is not something I'm trying to protect from the user, only from malware. Thanks again for all your help.