I think the only through bolt you would need to use is for the most significant shroud. Perhaps a lower as well. You would use a stainless bolt smooth on the shank and only threaded at the point is exited the mast on the opposite end of the entrance point. A nut with plastic securing material inside (I forget what they are called) would be plenty to hold it. Besides, your concern is actually the silver lining. In short order, all you stainless fastening with "cold weld" to the aluminum mast. Your standing rigging
is what holds the mast up, not the mast as anyone who ever had a shroud break knows very well.
Everything else can be stainless machine screws or stainless sheet metal screws. Someone will have a better opinion on that than I. Backing plates
? I can't think of why you would need those. Just keep in mind the different between 0 degrees sheer force and 90 degrees pull out forces.
If you use a light pole as someone suggested, you would need to attach a sail track. This would need to take 90 degrees pull out forces. Track overcome that problem with many very close fastening, but I don't think you are considering this approach. Any extrusion you buy designed for sailing will have a track set up for sail slides.
So concentrate on rigging. The mast is just an excuse for a place to attach the standing rigging and the sails
As for sleeving it in sections. This must be done routinely in the boat
building industry. In fact, I have seen masts bolted up in sections. No big deal I guess. And the idea of transporting the whole thing in small sections may have some appeal depending on your logistics.
Basically you are just building a series of mast steps. As long as the walls are in alignment I can't see any problem with building up your mast height that way.
Overbuilt, as some responders have suggested, doesn't really help, in my opinion (amateur though it is). All you do it create weight aloft and a feeling of psychological well being. Overbuilt when it comes to the mast may actually be contrary to the real objective, safety
The way I understand the whole business of mast and rigging is as the wind
increase and the boat
heels (cat or mono) the force downward on the step becomes enormous. The forces operating to break the mast into pieces are much less by comparison and minimal if the rig is set up to prevent the mast from pumping either fore and aft or port to starboard.
Using a smooth through bolt on a mast the size you want, internal halyards won't even notice it.