On the side of the lens board there should be manual F stop . so you can stop down and use open flash , though you may well find there are differences between the manual settings and the copal shutter
Are you using DB or DBM mounted lenses? The DBM lenses allow you to set the aperture independently as well as via the Auto Aperture Shutter. If you are interested in using them without the shutter (for a scan back, for instance), it would behoove you to find DBM mount lenses whenever practical (or alternately, DBM mounts and exchange your elements).