It's not totally irreversible.
It's just destructive/lossy; with typical 32-bit RGBA encoding, you're not likely to get precisely the original colors back in pixels that weren't fully opaque, which are the edges, or any semitransparent or glowy bits. So you could reverse PMA to some extent, but it's still a better if you have the original source files. Maybe not noticeably better, but better.
As for popular programs that have a one-click option to reverse premultiply alpha.... I don't know any. You could probably ninja layers and blend modes in photoshop to do the math for you.
A programmer who can do pixel pushy things can do it easily:
The PMA operation (literally pre-multiplying the alpha) is:
r = r * a
g = g * a
b = b * a
a = a
So the reverse is:
r = r / a
g = g / a
b = b / a
a = a
(making sure to set the color to 0 if a is 0, because you can't divide by 0)
Nate did mention he wanted to add the feature to Spine "with a warning". Tracker is linked in that linked topic.
Actually, you know what? It wasn't hard to do in Photoshop.
Basically clipping mask a copy of the transparency/alpha channel to the original image, and set it to Divide blend mode.
You could just as easily do it in CSP or some other art program that has those same features.
Here are some Actions.