Quote Originally Posted by Max View Post
Is it safe to drill into the side of my (empty!) fire extinguisher bottle and tap a hole to have my outlet in the middle of the bottle?
As long it's empty , you can drill and tap to you hearts content.

Not sure if there is enough wall thickness or if it makes it lose a lot of the strengh?
If you ask that, you learned something. NO that wall ain't thick enough to do what you suggest.
For 1/8 or 1/4 bsp you can do so in the bottom - as it is about 3 times thicker than the wall, but even then you'll need to have it retested.

If you want a 1/2 BSP or bigger in the sidewall, you'll need a welded in/on piece. Most ali CO2 bottles are from a weldable kind of ali. And with the advent of cheap AC/DC TIG welders, more and more people can do so.

The main problem here is to get a firm retesting and certify adapted bottles.