This happen to me around 8 years ago with RBG, they let me go offshore from the heliport but when I got offshore the OIM told me I had to go off before it expired. RBG phoned the OIM and he told them it was his discretion and he wanted me off. When I got off I was 2 days overdue and RGIT told me I would have to do the full 5 day course, when I told them I had been offshore RGIT told me I should have got a letter from the OIM saying I was offshore, If I had had a letter I would have been able to do the refresher.
So all in all yes you can go offshore but get a letter off the OIM. This was 8 years ago by the way.