I would think that chambering a cartridge in a Mauser with the cocking piece uncocked would not be difficult, just hold back the trigger. However, in doing this, it seems to me that you would be running a risk that the cartridge would fire, if the bolt were closed smartly, and firing a cartridge in a rifle with the breech unlocked would not be a good idea at all. (After posting this I realized that the firing pin does not fully protrude from the bolt face until the bolt handle has locked down, if the bolt is closed while simultaneously pulling the trigger, but what I have to say about carrying a live round in the chamber with the bolt closed and the striker down still goes.)

Even if the cartridge didn't fire, carrying a loaded rifle uncocked with the bolt closed would be running the risk that if the cocking piece were hit smartly, say as a result of dropping the rifle, it would discharge. The wing safety on a Mauser is about as safe anything can be, with the firing pin positively blocked, so there is no need to carry a rifle with the striker uncocked as a safety precaution.