Gear that is equipped should be locked automatically IMO, and there should be a lock function for other gear.
Not a good idea per se. If you lock a staff because it's equipped on your 6 mages, you'll need to load each one of those to check on which one it's used and remove it. You might have to do it as much times as you have mages. Now imagine that nice ring on all your characters...
On the other hand, marking an item as "important" could ask to confirm twice before salvaging it. Still, it's not solving anything as you can salvage it by accident anyway. In the end, if you're not paying attention or do something you don't understand without asking questions first, you'll pay the price. It's not anything different from real life...