I could have sworn that the Delta Reserve Companion Ticket could only be used for A and I fares, but I noticed that the terms and conditions now read as follows (emphasis mine):
Tickets are only available in I and Z classes of service for First Class travel, only available in W class for Delta Comfort+ travel, and only available in L, U, T, X, V classes of service for Main Cabin travel and may not be available on all flights or markets.
Terms, Conditions & Disclosures
I tried making a dummy booking for a RT itinerary with Z inventory of at least 2 on both legs, but delta.com would only load the I fares when applying the companion certificate. Has anyone successfully used their companion certificate on a Z-fare?