In regards to BFEL's question and Putnam's response:
I believe that Putnam is correct - getting guns to show up during gameplay-worldgen (we need a name for this... succession?) without having been in worldgen won't work (unless something has changed significantly), but not for he cited reason. In that quote, Toady is talking about interactions/abilities like vampirism. Presumably guns are a weapon, not a skill, so the issue is elsewhere.
The problem is that a weapon type "showing up" is decided by the entity raws, which are copied into the world save at the start of worldgen. There are some workarounds to get it so that your fortress alone could produce guns post-worldgen, but actually getting them into the hands of a civ (so you can see people out on the street with them) is a different matter.
This might change whenever Toady updates the economic code to be a little more reasonable with materials available and goods produced, since right now, the game isn't very good at having civilizations produce goods based on raw-defined reactions. Right now, dwarves always have steel, even when the civ has no access to iron or flux (in addition to a whole bunch of other strangeness), plus weapons are not produced by reactions (not at the vanilla forge anyway).
So, barring someone doing something really sneaky and clever I haven't thought of (or hacking your savefiles somehow) this sadly won't be possible in the upcoming release. As I said though, once some more economic stuff gets moved over to the raws, the issue will likely be resolved.