Knowing Irwin Allen, he never would have gotten to "but not prohibitively so" before hanging up the phone.
That's just it, though, in a forced prospective shot, there isn't any optical printer work required. Right? Unless I am misunderstanding the process, which is always possible, the actors stand at different distances from the camera, the sets are matched, and then shoot, cut, print. Wouldn't that be easier and faster than shooting the scene more than once and then sending it to the lab? Especially for an episode that was already heavy on new effects. The majority of the physical effects on Voyage were done in camera and pretty simply.
This is why I'd love a really good behind the scenes book on the series. I love going over this stuff.