If the additional interpolation isn't a concern, then you can do the 3drotate command. Slice timing correction (3dTshift) would have to be avoided if you do a 3drotate first. Alternatively, you could align to a template that is AC-PC aligned or separately rotate an anatomical dataset (which could be your FLASH dataset) and align the EPI dataset to that. If you're doing motion correction anyway, you might want to combine the transformations as done in our standard scripts, afni_proc.py and align_epi_anat.py.