The way of handling the Fuji 645 for landscape mode is exactly the same as RF645 (you need to keep the rangefinder "up" for focusing).
The other thing for this default vertical orientation is the flash. If you use flash, then the flash needs to cover the entire vertical frame in portrait mode, while most flashes are landscape by default. So you need to use a wider-angle setting on the flash. Or like Fuji GA645, you can use its dedicated bracket that can rotate a mounted flash, or its dedicated flash unit whose default coverage orientation matches GA645's.