My experience of flagpole suppliers and manufacturers is that they run away when you talk about using their products for mast. I think you need to do an independent assessment based on the specifications. The section should be spun-tapered for a start, not welded, and the wall thickness should be a minimum of 5mm for a mast suitable for your boat. It should ideally be 6061 alloy with a T6 tempering, for additional hardening. I don't think my 10.5m mast on Arion was T6 though. It had a diameter of 200mm at the base and began tapering about four metres up from the base to 100mm at the truck, with a wall thickness of 5mm. It was the bottom section of a 25m flagpole, made in France. Setting 35 sq m of sail, I hammered that rig for some 15000 miles of coastal cruising without problems. It is hard to say if an alloy or timber mast would survive a violent capsize, which is always a possibility in the Southern Ocean. If I was sailing there with an alloy or timber mast, I'd carry a built in jury rig on deck, like Galway Blazer did.
Lady Pepperell, a boat with unstayed carbon-fibre masts, was violently pitchpoled in the Southern Indian Ocean in the 1982-3 BOC race. Her rig survived, even though the force of the 'corkscrew' capsize caused severe structural damage to her keel, and the boat was abandoned. I'd choose a CF mast for the south, if I had deep enough pockets. Timber and alloy masts are perfectly ok for most eventualities, however.
PS: If I do another junk rig conversion, I will probably make a hybrid mast, with an alloy bottom section and a timber topmast. I have sourced a straight 6.5m section with 200mm dia and 6mm wall thickness that would suit. I'm not planning to go into the Southern Ocean, however!