I read more of the GlobalPlatform documentation and understand now why the ADF.USIM is selected first (and why it should be first). The applet doesn't need to be marked as default - it just needs to be the first "SELECTABLE" among the list.
Attempting to modify the USIM application to run what I need doesn't seem realistic either :/ If I figure anything out, I will share it here for others as well.
