For BI (RTW is similar) look up the general (not named character) section in descr_character.txt eg:

faction alemanni
dictionary 15
strat_card data/ui/roman/portraits/captain_card_julii.tga
strat_model BI_sm_captain
battle_model saxon_general
battle_equip gladius, chainmail shirt helmet and rectangular shield
that tells you what the strat_model is, look up the strat_model eg:BI_sm_captain in descr_model_strat.txt and you will see what texture is used for your faction.
type BI_sm_captain
skeleton strat_named_with_army
scale 0.7
indiv_range 40
texture alemanni, bi/data/models_strat/textures/BI_strat_captain_alemanni.tga
texture empire_east, bi/data/models_strat/textures/BI_strat_captain_empire_east.tga........etc
you can obviously direct it to new model/texture so long as they exist.