How is the default screen orientation in android specified?
It's possible to override the default screen orientation by disabling accelerometer rotation and setting user rotation in the system settings database. The phone/tablet needs to be rooted to do this. The change is applied after the phone/tablet is rebooted.

On the console:

su
sqlite3 /data/data/com.android.providers.settings/databases/settings.db
insert into system (name,value) values('accelerometer_rotation',0);
insert into system (name,value) values('user_rotation',1);
.exit



user_rotation 0 -> ROTATION_0
user_rotation 1 -> ROTATION_180
user_rotation 2 -> ROTATION_270
user_rotation 3 -> ROTATION_90


See http://developer.android.com/reference/android/provider/Settings.System.html#USER_ROTATION for more details