Firstly a disclaimer, I'm not an instructor and I'm not a helicopter pilot. Also, I've not looked any of this up, so treat what I post accordingly.
However, I do hold JAA and FAA PPL(A)s. (Ok, FAA PPL(SEL) and JAA PPL(SEP)).
In my experience the position is:-
(1) As previously stated, hours is hours, but
(2) JAA will not accept hours by an FAA instructor as hours with an instructor, unless the school is specifically approved (normally by UK CAA) for JAR training.
(3) FAA will not accept a JAA biannual "flight with an instructor" as equivalent to an FAA BFR
(4) JAA will not accept an FAA BFR as equivalent to a JAA biannual "flight with an instructor".
(5) FAA will issue a PPL(SEL) on the back of a JAA PPL(SEP) with no additional flying beyond a BFR. Unless you have a separate night rating on your JAA licence, you get a "no night flying" restriction on your FAA PPL.
So, with my plank licenses, I have no problem with currency / total hours, but have to two biannuals (one FAA and one JAA).
How relevant this is to rotary, I've no idea, but it might help.
G