damage from continued exposure to the reduced air pressure in the hold
Do any pressurized aircraft have an unpressurized hold? The hold pressure will normally be very close to the cabin pressure. If that were not the case, the aircraft would not be cylindrical.
As mad_jock says, it's more likely to be the temperature that causes a problem. Dive computers are perfectly fine to be taken to altitude.
Mine specifies a 'normal altitude range' of 0-10,000ft AMSL, an operating temperature of 0 to +40 C and a storage temperature of -20 to +50 C. I don't think you're likely to exceed any of those limits for an item in a bag on a commercial airliner for a typical flight duration. If in doubt, take it in hand luggage.
Even above 10,000ft cabin alt (e.g. unpressurized light aircraft) I still wouldn't expect there to be a problem but my Suunto manual has no guidance on that.
Mark