Tested it some just now.
These work:
- computer
- tablet
- smartphone
- speaker
- tv
- avr
- stb (same icon as speaker)
If set to any of these, the device no longer shows up on Spotify Connect:
- audio_dongle
- game_console
- cast_video
- cast_audio
There should be one more device type, resembling a speaker group. Not sure if it can be achieved though
