Although I have yet to try this method, others on this forum have done it very successfully (not lose the bonus OBC). The key thing is to make sure you move your reservation to the transatlantic when the time comes. Do not cancel the dummy reservation and then rebook. If you preserve the original booking number, your onboard booking OBC will follow along with it. the original booking number is preserved when the booking is transferred while cancellation and rebooking does not.
It also stands to reason that you will want your dummy booking to be a refundable deposit booking to avoid the change fee when you transfer it. When you transfer it, you can decide either to keep it as a refundable deposit or convert it to a non-refundable deposit for the new sailing.
The exact timing that the eastward transatlantic sailings will be released is not known. Maybe it will fall within your 6 month window. However if it falls in December like it did last year, you have a good option.