Rarity doesn't track salvage crates. I think it would be possible to just change the detection method to USE for Island Expedition items? Those items currently use SPECIAL and some custom event handling to add attempts after every run. Not sure if there'd be side effects to changing this. Also, IIRC there are complex requirements for the drop itself that aren't really being captured.

One wrinkle: The drop rates are (obviously) very different, with no way to differentiate currently.

With drop rates on the order of 5-10% (according to wowhead data), but Rarity currently using 1 in 1000, and the real probability being unknown for expeditions themselves - but scaling by difficulty... I don't have any hope for getting it right. So the best worst solution might be to just leave it at that and accept that anyone who opens crates will see a wrong drop rate.

Edit: The alternative is to set it to 5-10%, but then people might assume that these collectibles drop frequently from expeditions.