Hi Stone,
cassettes are reusable as long as they work - do not let light in or damage the film. 70mm cassettes are basically giant brothers of 35mm ones. They only have different spools, outer shells are very similar. I'm using Alden type loader (thank you Matt!). I insert spool in the dark in the loader, tape film for the spool, close loader, turn light on, wind certain length, turn light off, open loader, cut the film, insert spool in the cassette. So there is no waste of exposed film. Used plastic ORWO cassettes for first 15 years and then switch to metal ones "Agfa style". Recently when I started using some new cassettes I started marking them after each loading. Always keeping cassettes in plastic containers and keeping them away from dirt and dust, as much as I can.