I have reason to believe I can give rebels their own banner, depending on how they are defined (Clan-specific rebellion, like "Oda Rebels", or religious rebels; "Buddhist rebels"). First, I want to put forth evidence to the point:
In local_en_patch.pack > text > db > factions.loc there is a distinction for different kinds of rebels: "factions_screen_name_when_rebels_<faction_id>" and in the second column the particular name for that.
The second and more important bit of evidence is in "banners.tai". Using "banners<idx>.dds" (four dds files in total) the banners.tai file creates a sort of artificial .tga file (there is probably a better term for it, so forgive me there). One of them, for example, is named "mon_stratbanner_matsuda_diffuse.tga". At first I thought the "matsuda" bit was a sort of hook (again, probably a better word out there) to the Faction ID, but this proved incorrect as there would be two Urakami, for example, .tga files, one being called "mon_stratbanner_urakami_diffuse.tga" and the other "mon_stratbanner_tut_urakami_diffuse.tga".
Edit: Oh, I completely forgot this bit. All rebellions based on major clans (for example, Chosokabe rebels) have that major clans' banner. That means rebellions use more than one banner!
With that, I believe I can do this. The small problem is that I haven't yet been able to find any sort of reference in the game files to these .tga files. The closest I have come is Model filepaths in "warscape_rigid" and "warscape_rigid_lod", wherein they referred to rigid models like "sho_char_flag_uesugi.rigid_model_v2". The relevance of this, however, can be refuted if you consider that there are only ".rigid_model_v2" files for the playable factions; any relevant reference would be that to all clans, not just a minority.
I figure I am simply not perceiving the relevant db file, so I will keep looking, but if someone could actually help, it may drastically reduce the time I commit to finding that reference, assuming I am on the right track; even longer if I'm not.
Edit: Ok, this might be a dead end.