OK, I just looked at the build sheets in the Tech section here. I don't see anything in the remarks section on any of them about changing seats, other than reinforced seatbacks on the 1984 and 1985 sheets.
http://www.specialservicemustang.net/Build.html
The last sheet on that page (1993 FHP, PF154606) has "G6" for the trim code. Other FHPs in the VIN Project with the same DSO have "D6" for their trim code on their respective door tag.
Looking at the next two build sheets up from the bottom (1992 FHP and 1992 CHP) I see "FA" trim code for cloth seats.
The next one up (1992 Gainesville FL) has "DA" trim code, and the Marti report on its VIN Project page also shows the same trim code.
I will have to look at the database later and see what kind of door tag code and buck tag code patterns emerge.