I don't see a problem welding up a 100psi air tank for a pinhole leak. Not enough pressure to worry about.
Presuming an old-school wheeled shop compressor of ten gallons or so.
Ten gallon cylinder will be roughly 12" diameter / 30" length
= 1131 sq in cylinder surface area
= 226 sq in end cap surface area (113 x 2)
= 1357 sq in total interior surface area
@ 100 psi = 135,700 lbs total
@ 100 psi = 67.85 standard tons
Yes, it is something to worry about. We're discussing a tank with enough internal rust to manifest as an external blemish. Rust that's penetrated from the inside out is like an iceberg. What you can see is a small portion of the total. There is significant involvement of the area around the detectable leak. That means the structural integrity of the entire cylinder is compromised.
A tank failure could easily launch a fair sized chunk of metal with enough velocity to do serious bodily harm. That much pressure contained in a cylinder with obvious structural integrity issue = Nope. For reference, a champagne cork experiences roughly 80-90 psi. The slow-ish pressure release of carbon dioxide from its liquid host also imparts a relatively short pressure pulse, much shorter than a container of compressed gas. Finally, a cork is a relatively forgiving projectile in terms of both weight and density. Even so, plenty of folks walking around with eye patches.
Now imagine the same scenario, but with a razor sharp shard of rusty steel at greater velocity. Ditto having a wrench or hammer blown back at you.
Brad