There are a few ways to get random faces on to custom soldier models.
1) Rely on the BIS random faces using the osobnost selection then using a setface script to set the random face on it. (maybe enable the script via the init eventhandler in the CPP)
2) Use the hiddentexture/setobjecttexture method, however this is bugged in that the setobjecttexture face disappears after a saved game is restored.
Method 1) is your best bet for a stable useable addon.
SelectThis