If I remember correctly, when you have the FAA licence based on a JAA or any other foreign licence, the FAA logo in the top corner is black, not blue, meaning it has condition attached to it.
When this licence is issued, there will be a letter that accompanies it. This letter has a 6 month expiration. If you do a FAR 141 IR (not a part61) within this time frame, you can then add an the rating to your certificate.
If the letter has expired, then you will need to re-apply for a new letter from the FSDO.
This is from memory. Don't call a FSDO, because they know less about licencing than FCL at gatwick. You get a different answer depending on who you talk to.
Call a school, and ask them. They can always ask as DPE, who will be the issuer of you licence. Ok, they call it a temporary airman certificate, but it is more than a pass slip, it is the licence, and if yo have applied for the test through the new computer based program, there is no later option to deny it, lice the CAA can do.
So thats what I remember. I may be wrong, but I don't think so.
IBR