Greetings,
The ARINC 424 will not solve any problem since it is the ADRU (Air data side) that needs to be modified, todays solution is, instead of computing a height difference and applying it, we translate the height difference in distance and move forward the FAF, since at you "plateau" altitude you benefit from maximum terrain protection, in both TERPS, and PAN OPS worlds.
Delta ISA X 4 X 1000s of feet.... delta isa =-15 FAF altitude =2000
-15X4X2 = -120 Feet 2000-120= 1880
120 Feet will give 0.4 NM so either you descent to 1880 Feet, or you maintain 2000 and start descending .4 nm after the FAF