I have researched this in the past; so not sure if the info I have is up to date.
An FAA ppl and multi can be issued on the grounds of your JAA licence, it is in effect a paper work exercise (can't remember the exact process).
If you have a JAA multi IR, then there is a FAA foriegn pilots written IR test, this test can be taken in this country (UK) and will allow you instrument priviliges for private purposes in the US. So you can get the FAA private multi IR without taking any flight test.
Some schools will have insurance minimums for hiring a twin (ie: 50-100 hours multi), however 5-10 hours checks outs are a common requirement.
As Big Grecian points out, when you see a hundred hour block advertised quite often ony 50 hours can be logged under JAA rules. However Ari Ben use to give an option of paying an extra 20 dollars an hour on the advertised rate, in order to allow you to fly all hours as the actual handling pilot.