It's much simpler:
In Germany the GEMA quite recently closed a deal with YouTube that allows YouTube and its users to use all GEMA material in videos uploaded to their platform. YouTube is paying an undisclosed annual lump sum to GEMA. Also, GEMA represents not only the rights of their german members, but those of the members of most foreign copyright societies, aka: worldwide, too.
Furthermore, german Urheberrecht says if a musical work has been performed in public, the copyright holder has no right to prohibit any further performance of the same work in its original form, as long as he gets paid the royalties for the performance.
So, given that Ligeti was a member of the austrian AKM, in Germany I would simply upload the rendition to YouTube, as YouTube is paying for the right to use his works.
As the GEMA was the last copyright society in the world to finish a deal with YouTube, I would strongly assume that the societies in Austria, the USA or Italy have similar deals.