A plug right above the internal downhaul system is pretty common.

My solution would be to just drill a small hole just above this plug and drain the water water out that way. Possibly by drilling a second hole 100 mm higher up the mast (to let air in more easily). This should be about 1500 mm up from the mast base BUT PLEASE CHECK THIS with the builder of your boat/mast.

These holes will be in a pretty safe place as this part of the mast will never submerge in a normal capsize. Only when the boat goes turtle but that is an extremely rare occurance with the F16's due to their overall lightweight and relative bouyant masts. So one could leave these holes open and allow the mast to vent. This also avoids underpressures in hot masts when they are suddently cooled by the water during a capsize. This underpressure can be a cause for the water inside the mast.

Anyway, but if this is not to your liking then putting two aluminium blind rivets (pop rivets) in will seal the mast again. Aluminium rivets are also very easy to drill out when you'll find water inside your mast at a later time, thus reusing the holes. Alternatively, you can place a screws into the holes or just take them off.

Wouter