I did this combining save files thing and the "imported" characters _did_ get portraits if you manually edited one value (portrait_index?) to something not in use by the old characters. Granted, I was working far below that 256 limit...
That's portrait_index, yes, and it is limited to 0-255. Characters after 256 all get 255 too, and editing it beyond this value doesn't change a thing. The generated portrait texture is 16x16 portraits.
Why would you need to combine save files anyway? Between randos at bars and exploiting slave camps you can easily hit 256 recruits without any shenanigans.