Yes! You can definitely fullscreen Lumen to another monitor. To make it work, you must disable "Screen Mirroring" on your Mac in order for it to see the HDMI output as a separate display:
- Open System Preferences.
- Select Displays.
- Select Arrangement.
- Uncheck Mirror Displays.
Then you can select the external display from Patch -> Fullscreen, and set the settings for it under Patch -> Fullscreen Options.