Let me sum it up:
UK and US: virtually impossible.
Rest of the world: Very difficult, but not impossible. Bringing meaningful credentials may help, and being very lucky as well.
And that fits my experience.
Pre 9/11: Dozens and dozens of LH, BA, Spantax, and several others.
Post 9/11: About 20 jumpseat rides on A320s, Dash 8 and Do 328. No chance with LH and BA.
Strange times we're living in...!