There is no technical reason why using a camera as a light meter will not work fine. A few people report success with this.
You will get differences of exposure between different cameras because they use different "intelligent" metering methods and will therefore react differently to the same scene (although you would expect them to be within half a stop or so of each other when metering a "normal" scene). If one of your cameras has a "spot" metering mode, use that with a long lens (to make the spot as small as possible) and use it as a spotmeter. That will get around any confusion caused by the camera's "intelligent" metering modes...
You do not need to use equivalent focal lengths: 1/60th @ f/8 is 1/60th @ f/8 with any lens (of course, you need to set the same film speed too). The only real downside is the bulk and weight..