Are the cases for the inserts on the M645/1000s light proof? They don't seem like they are, but it seems like they should be. Of course changing inserts would have to be done in a changing bag, but I'd like to know if the option is available with the 'carrying' cases for the inserts.
No, but they protect enough that you can leave in them unexposed film advanced to the start mark, or exposed film advanced to the end but not yet sealed with the tape.
Their purpose is merely to allow you to pre-load several rolls into several inserts, so when you finish one roll it is a simple and quick matter to roll it to the end, take out one insert, pop in another insert, and then advance from the Start mark to the first exposure.
If you have a power winder, you can do this really quickly. Even when you use a manual wind, it is quick.
Great for weddings. Just be sure to develop a system to keep the unexposed film inserts separate from the exposed film inserts!