I also remember coming across some other interesting properties in frameworks/base/services/java/com/android/server/SystemServer.java, a quick grep yields:
String factoryTestStr = SystemProperties.get("ro.factorytest");
final boolean headless = "1".equals(SystemProperties.get("ro.config.headless", "0"));
boolean disableStorage = SystemProperties.getBoolean("config.disable_storage", false);
boolean disableMedia = SystemProperties.getBoolean("config.disable_media", false);
boolean disableBluetooth = SystemProperties.getBoolean("config.disable_bluetooth", false);
boolean disableTelephony = SystemProperties.getBoolean("config.disable_telephony", false);
boolean disableLocation = SystemProperties.getBoolean("config.disable_location", false);
boolean disableSystemUI = SystemProperties.getBoolean("config.disable_systemui", false);
boolean disableNonCoreServices = SystemProperties.getBoolean("config.disable_noncore", false);
boolean disableNetwork = SystemProperties.getBoolean("config.disable_network", false);
But those implies you have already started the zygote.