I can't argue with Cusco conclusively (that is, I can't provide specific references to FAA regulations on the subject).
All I can say is that when I did my IR last year (at the time I was on a 61.75) I was told that in order to meet the IR requirement...
"A person who applies for an instrument rating must hold at least a private pilot certificate.."
...I must have met the requirements for an FAA PPL as this was technically what I was adding the IR to - not my JAA licence. On the day of the test the examiner also checked my logbook to make sure that this was the case.
The following link makes reference to a similar check so I am not the only one that has come across this. Hopefully somebody can provide a conclusive answer one way or another;
How to get an FAA Instrument Rating