It is still possible to take the written test for the "instrument foreign pilot".
from 14CFR61.75
--------
(d) Instrument ratings issued. A person who holds an instrument rating on the foreign pilot license issued by a contracting State to the Convention on International Civil Aviation may be issued an instrument rating on a U.S. private pilot certificate provided:
(1) The person's foreign pilot license authorizes instrument privileges;
(2) Within 24 months preceding the month in which the person applies for the instrument rating, the person passes the appropriate knowledge test; and
(3) The person is able to read, speak, write, and understand the English language. If the applicant is unable to meet one of these requirements due to medical reasons, then the Administrator may place such operating limitations on that applicant's pilot certificate as are necessary for the safe operation of the aircraft.
--------
Also there is some more info in the FAA examiners handbook, Chapter 29
http://www.faa.gov/avr/afs/faa/8700/...2/2_029_00.pdf
--------
Note that it says "... on a U.S. private pilot certificate". I don't know if this means that there would be a restriction on a commercial license saying the iinstrument priviliges can only be used as long as your are flying on private, and not commercial priviliges. Best way to check, is to call a FSDO if you want to know for sure.
The words "US test passed" will only be put on a license when it is a FAA license based on a foreign license and you have passed a std instrument checkride. Since
Long final already has a FAA stand alone license, this is not applicable
Personally, if you are going to take the written (which is almost the same as the standard instrument written test) I would take the standard test, and do the checkride for the normal instrument rating, saves you alot of hassle.
The only thing what you have to look into, is how it works, as far as TSA clearances go, with adding the instrument rating.
-IBLB-