Whatever technique was used, it (unfortunately for us) seems to be too commonplace for the Chief Constructor to mention it in the chapter on iron masts in "Shipbuilding in Iron and Steel" (from page 259 & table 7 on page 522) **LINK**.
Hard to imagine that any kind of dolly or hydraulic rivet press could be inserted from the end to the full length of a section, especially if there was internal bracing as described in the book. Perhaps the back of the mast was supported so that a jam back could be used to snap the rivet?
Brian