Do most airlines allow a passenger to book two seats for any reason? The obese passenger and the cellist would both fit into very specific ticketing policies. (I think some airlines have killed off the ability to book seats for musical instruments.)

If the policies limit who can buy a second seat, then the natural response from the airline would be to just ticket the infant in the second seat.

Given how many European fares have very low base fares and very high YQ, I'd be surprised if this actually works in practice. If it does, people have been really quiet about it.