Bill, most Super 8 cameras have beamsplitters in the light path. The Nikon R10 stands out among them for light lost between the front of the lens and the film.

Beaulieu S8 cameras (except the 1008 and 1028) have reciprocating shutters with a mirror. With them, when the shutter is open all of the light that the lens passes goes to the film. When the shutter is closed all of the light goes to the viewer; light for the meter is taken from the viewer by a beamsplitter. Y'r explanation doesn't work for these cameras. My dim Angenieux 8x8B was on a Beaulieu 4008 ZM.

Canon's 8.5-25.5/1.0 zoom, as fitted to the Canon 310XL, 310XL-S and AF310XL cine cameras was probably the fastest photographic objective made in (relatively) large numbers. Canon were very serious about making these cameras as usable as possible in dim light. To this end, the 310XL had a non-TTL meter, a relatively slow shutter speed for an S8 camera, and fed light to the viewing optics with a tiny fixed mirror (not beam splitter) behind the lens. If you look at the specs here you'll see that they assert the 310XL has a TTL meter; this isn't so, the meter is external, with the cell located above the lens.