As I'm still not entirely sure which files originated from what, I can only reference my current setup, which is the pi-to-jamma mentioned above. The key to forcing 240p for Attract Mode was to modify:
1) /boot/config.txt (this should've been more obvious to me, as the file itself points this out)
hdmi_group=1
hdmi_mode=8
2) then modify all /boot/launchers scripts and comment out the last lines, e.g.
# resolution back to 720x480i
# tvservice -e "CEA 6 HDMI"
# fbset -depth 8
# fbset -g 720 480 720 480 16
This has made Attract Mode render in 240p on my 15khz CRT, even after quitting a game.