Am I missing something obvious? When did this get so hard?
In the old days, you configured your desktop session on a Linux system by editing the .xsession file in your home directory. The display manager (login screen) would invoke the system-wide xsession script, which would either defer to your personal .xsession script or set up a standard desktop environment. You could put whatever you want in the .xsession script, and it would be executed.