If the mast wall is too thin, you can't trust a tapped hole to have any serious strength. The mast on my boat is such that the spar manufacturer said that nothing should be attached using screws, since they won't have a prayer of holding under any load. The winch bases and cleats are either through-bolted to a backing plate inside the mast or screwed to a base that is riveted to the mast.
I'm surprised the spar maker would say that. What size of mast is that thin?
Here's part of the article "Installing Hardware on Aluminum Spars" from DIY Boat Owner magazine (1998 issue #1 page 27):
Machine screws are used exclusively by custom mast builders to fasten most hardware. More time consuming to install than rivets, they require drilling an undersize hole, then using a tap drill to make a threaded hole for the screw. Screws are identified by the size and number of threads. A No. 10-24 screw, for example, denotes a screw with a maximum diameter of 10 (.190) and 24 threads per inch. Look for National Fine (NF) screws; the finer the thread, the better the holding power.
In this article they interviewed Danny Klacko of Klacko Spars. His company has built spars for a number of production boatbuilders, including for all the C&C boats before Tartan bought the name. C&C were high performance boats, so I would expect their masts to have been as thin as possible.
My previous statement about using coarse thread screws in aluminum stands corrected, although I'm puzzled by their example because 10-24 is coarse and 10-32 is the fine thread in that size.
I've got Klacko spars on a 1976 34' sloop (and a 1973 34' ketch, but that's another story). The boom was assembled with rivets, and much of the mast hardware with machine screws. When I got this boat most of the rivets on the boom were loose. All of the machine screws are tight with none of them showing any signs of working. The gooseneck was worn out, so this boat did do some sailing in that time. The mast is probably 1/8" thick, which is typical. There are no backing plates.
SD - the OP asked about putting steps up a mast. If a mast is so thin that machine screws would pull out, wouldn't you be concerned about the mast dimpling around each step if an adult climbed the mast? Those dimples would really weaken an already fragile mast.