Dol Guldur,
really didn't do anything special,
I'd got to launching vanilla BI stage by making new folder in RTW/BI called 'modname' and pasting entire (BI) data folder into it, if vanilla BI is working off shortcut I assume you got to this stage.

- I already had my mod in a zip folder designed to overwrite a vanilla BI install i.e with data/ structure already in place containing all changed files. I just copied my mods data/ folder over R-TW/BI/modname/data to overwrite files - that got all icons / banners / maps in without any further editing. I just had to change path to textures as they where modded for strat map captains etc. which gave -show_err message during campaign loading bar.

Seems to work regardless of whether campaign folder is named for provincial or main campaign.

Sounds rude to suggest this; but are you sure data folder you put your mod files in is the one the path name is refering to. Also if your approach was similar to mine did it actually overwrite files or did you get modname/data/data?
(yes I missed extraction path and got that first time out)