Is it possible to find the full SELinux ruleset for OEM's stock ROMs?
Yes, but it depend where these policy files are stored. Many manufacturers like to move stuff around, thinking their own security solution is better. On top of that depending on you AOS version, things may have changed again, since SEAndroid is a quite recent implementation. 
One place to look for these files is at the Android Census site. There you can navigate to each device policy/context file. For example:

/seapp_contexts:

isSystemServer=true domain=system
user=system domain=system_app type=system_data_file
user=bluetooth domain=bluetooth type=bluetooth_data_file
user=nfc domain=nfc type=nfc_data_file
user=radio domain=radio type=radio_data_file
user=_app domain=untrusted_app type=app_data_file
user=system name=com.sec.knox.containeragent domain=knox_system_app type=system_data_file
user=system name=com.policydm domain=knox_system_app type=system_data_file
user=_app name=com.centrify.mdm.samsung seinfo=knox_untrusted domain=knox_untrusted_app type=app_data_file
...