You need a booking system that allows you to select a particular fare - not just fare class, fare basis as well - which basically means you need to have GDS access or someone with GDS access. The company that I know that buys these fares uses a TA that is in on the game. Generally, it would also be a TA that would advise this method when consulted by a company looking for a solution to a buy-Economy restriction.

As I mentioned, there is always a companion Y-fare filed together with the Y-UP fare. That Y-fare is coded exactly the same with the same restrictions as the Y-UP, with the difference that it books into Y instead of C and that it is a bit cheaper. As a deliberate side effect, this always prevents the Y-UP fare from showing up in the majority of booking engines since most just go for lowest price, effectively hiding the Y-UP from most public views, even if Y/C are the only remaining booking classes available. That doesn't mean the Y-UP can't be booked though, you just need to be in on the game and know that it is there.