I would consider ultrasound inspection
The trouble is, you don't know how far the thinning extgends. A 1/4-inch hole may not be important at all, or there could be a large thin area. Ultrasound can also tell if the inside is smooth or rough. Of course, I do UT inspections and so this is free for me.
I would also be very surprised if the bandaid carried any important amount of load; compare the amount of missing area to the cross section of the rivets; it would take a LOT of rivets, close tolerance on the holes so that they carried load at the same time the mast began to deflect, and require a lot of drilling. Just a little slop under load, and the mast will begin to tear before patch loads up.
Is it a high load spot, and is it in compression or tension? Most masts fail by buckling up near the spreaders, not at the deck. A small hole at the deck in compression is probably unimportant.
(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")
"Well, I just climb up to them."
by Joe Brown, English rock climber
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