I would use a digital camera, guess at the exposure, take a shot, review and make adjustment and take another shot until I got the result I want chimping. I would have to learn to judge the exposure on the LCD screen, mental compensation for any differences between the particular digital camera and film and then make setting on the film camera and make the shot. This way it works well for me both with ambient light and with flash.
I would never use a digital or another film camera as a meter per se. The reason is that they are not as accurate as a good hand held meter or at least don't have the resolution of the handheld meter. They are cumbersome to use and the readout only in aperture and shutter speed. Sometimes a readout in EV is more convenient. Cameras don't have incident mode and their spots are not well defined. Unless you use a relatively long lens, most cameras spotmeter angle of measurement is larger than 1 degree.
As for the iphone meter apps, I don't have an iphone so I can't say much but I have doubt about its accuracy and I believe it's only reflected light.