In all FAA IR cases I know of, mine included, the previous IMCR training had been accepted by the US based school and its US based DPE without any problem, but in all cases I know of it turned out to be irrelevant for logbook filling purposes because more than 15hrs were subsequently flown with the US based FAA CFII in order to reach the checkride standard.
It is virtually impossible for an IMCR holder to reach the FAA IR checkride standard without flying at least 15 additional hours with the eventual instructor (who for equally practical reasons will be an FAA CFII) so this whole question is moot for the purpose of debating the legality of somebody's logbook.
This kind of thing (the FARs are silent on this specific issue) is well within the discretion of the individual DPE anyway.
The real benefit of using the IMCR towards the FAA IR is that the minimum dual instrument time is indeed just 15hrs i.e. the FAA IR works largely on the principle of "demonstrated competence". So an IMCR holder who is already very good can go to the USA and knock off the FAA IR in perhaps an extra 15-25hrs. Whereas if you did the JAA IR you would have to sit there for 50/55hrs no matter how good you were, but a previous ICAO IR reduces this to 15hrs. This is why the IMCR -> FAA IR -> JAA IR remains a reasonable route.