The biggest issue with spreaders is how they limit moving around on the deck when in the locks. If the spreaders are high enough off the deck, i.e., 6 ft. they should not be much of a problem. If they are vertical, again not much of a problem. If horizontal and below head height they will be a nuisance and will interfere with safe locking through.
In the locks you will be maneuvering in very tight quarters. And while you will try to stop the boat so that the person on deck is next to one of the lines or ladders in the lock, you won't always be successful. The locks have ropes or cables that you hold onto as you ascend or descend the locks. Some will be spaced so that the crew on the bow can reach one while the crew in the stern can reach another, but mostly, they won't be spaced that way. Crew will need to move along the side decks unimpeded.
In no way would I place weight on the spreaders. Spreaders are designed to take compression loads from the rigging; not the kind of load that would be placed on them if used as "legs" to support the mast.
The crane at Fair Point Marina (Fair Haven) can support the weight of your mast with the sail in it. You will be pushing the limits at Oswego. Hopenose (sp?) Marina near Albany should be able to handle your mast with the sail, not sure about the one at Castleton YC.