A Humm,
I think we have discussed this before but maybe some clarification is in order. I don't know how the Major Artists do it but I suspect songs are recorded all the time to see if they want to release the song or songs. If the song is not released for sale no mechanical license would be needed.

When songs are pitched to an Artist or Producer and they deem a song or songs to have potential for that Artist they may go into the studio and do a complete master recording of all the songs and then judge all of them to see what will go on an album and what will be passed on.

This could be called demo shopping if it is run by a Label head, etc.
In fact if you pitch songs to an Artist that is shopping a demo to a Label they can do that with the songs that they are considering without any permission from the copyright owner. It is only when they decide to release the song for sale that they apply for the mechanical license.

A song is not considered published until it is released for sale.
The mechanical license is paid, according to how many copies will be maufactured and what rate has been agreed to for the license to be issued.

Ray E. Strode