I think the greatest problem from reusing regular film cassettes (reloading the ones that you get with film in them already) is that once the end cap (the one without the longer end of the spool sticking through it) is that it's really difficult to get a good re-seal on it so that it doesn't pop off. If I was lucky I'd get a single reuse out of a cassette before the end cap wouldn't seal.
For some reason the reusable ones you could buy worked better at that. Their end caps fit better and seemed made to pop loose without using half your tool box to get the thing separated so you could pull the entire spool free at once.
I think the answer to the Green issue is to just buy reusable cassettes, use them as many times as you can and then send them to the metal recyclers to keep them out of landfills, etc. Besides, every time someone buys a digital whatever, before it's even out of the box it's headed for the scrap heap because the manufacturers are already in the process of outdating it with a newer (and not necessarily better) model.