Well, I hired Hurl and (provided he's got good Breath) he seems to be having a lot of luck with the crates on the island (at least on the eastern half). I guess Health + Breath is the big thing. It also proves he's good for more than battlefield surgery practice or, in Fidel's case, dog food. I also seem to have answered my own question!

Interestingly, the Crowbar halves the amount of Breath it takes to open a crate (at least it does on JA1 v1.13CD), even if a Merc could open it by hand. I wonder if a factor in the "crate opening formula" is the amount of Breath *after* one opens a crate; if the game calculates it as being too low (close to the Fatiuged level), it fails the test.

In addition, I have a 95% crowbar and a 50% crowbar, I have noticed no difference effect-wise in either implement. Maybe it is a factor on high-security crates, but not on the ones I have encountered so far in the game.