Previously on a shoot I noticed the DP taking photos pretty constantly during set ups with a DLSR (They were shooting 35mm). I was just a guest on the set so I didn't get in the way or ask questions but as I prepare for my first film shoot it occured to me that it might be a really valuable tool to bring along my 5D to take quick reference photos both for documentation of what a setup looked like, but to see how the light is playing at a specific ASA.
Would this actually work to give you an idea of what your exposure might look like? I know it wont be exact but if you are using similar ISO and Fstop should be in the right ballpark? Would just be a nice security blanket in addition to metering.