Inherit environment variable from launchd?

MacPorts’s “dbus” port defines launchd’s “SecureSocketWithKey” feature to create a temporary socket and assign it to an environment variable. This variable is inherited by all subprocess.

I’d like to create a similar environment variable (via launchd, inherited by all subprocesses) but with a static path. Is that possible?

I’ve looked at “EnvironmentVariables”, but that seems to be only variables for the daemon itself. I’ve looked at running “launchctl setenv” from a separate .plist file, but that doesn’t seem to be inherited by all of the session processes.

Thank you!