First class ZEDS are very very rare in spite of what "Google" search/posts.
The ZED fare are most "always" in the coach cabin - low - medium - high - with the MEDIUM being the norm.
Not all carriers participate with each other.
ZED fare Standbys are usually accommodated after the host carriers own employees but before other carrier non-revenue standbys and buddy passes.
ZED fares are in effect a revenue standby fare.
There are rules that are followed for clearance into a coach seat and the airline and its agents can upgrade to the First or Business cabin if coach is filled depending on agreements.

When traveling research the .COM of the airlines flying between the two points and see if buying a reserved confirmed ticket (for a few dollars more) is a better deal for your own sanity !
An advantage to the ZED fare - the baggage fees maybe waived - check that feature out !

Remember you are not traveling in the 'Glory Days' of aviation when a non-rev was almost certain to get on a plane and in First Class - flights are full and frequent flying revenue customers
are screaming for entitled upgrades - Welcome to the days of the back of the bus - if that !