There is a swivel arm for the Mamiya 7 which allows you to raise the filter to look through and adjust and then lower it onto the lens in the same rotational position. This might well fit via an adapter, depending on your filter size.
http://www.robertwhite.co.uk/mamiya.htm#LabelMamiya7
Otherwise, having two identical polarizing filters with clear witness marks on the rim, rotating one in front of your eye and transferring the setting to the other one on the camera would work reasonably well.

Regards,

David