IO540 is right on the money with his/her post - the only thing you have to be careful about with a JAA IMC/IR instructor is that he/she fully understands the FAA system. Although many things are the same and basic instrument flying can really be done by any instrument instructor there are a few differences from FAA to JAA that you need to know about. Remember; the examiner (if you can find one) will hold you to the FAA Practical Test Standard.
And at the end of the day you still need an FAA CFII to sign you off AND he needs to fly a minimum of 3 hours with you within 60 days of your skills test - and THAT requires a TSA approval - either way you look at it there is no way to get an FAA IR as a non-American without the TSA approval. The TSA clearly states that any license or rating that will further an applicant’s flying skills require TSA clearance PRIOR to any training being started. It doesn't matter if it's the first three or the last three hours of your course - you still have to pay your $100 and have your fingerprints taken. The TSA goes on to list these licenses/ratings as being the PPL, IR and the MEL. (note the CPL and CFI/CFII/MEI does not require TSA approval).
It is irrelevant if the training is done in G-reg or N-reg. The N-reg is only required for the skills test.
Happy Landings!