I'm starting to believe that the -mod:bi/modname actually just does the same as -mod:my mod in terms of getting a BI mod to work under BI.exe: ie. you need to copy in everything from the BI folder that's required for BI except things like textures that can have a defined relative path, or things that got duplicated into RTW with the post BI patches everything else I'm pretty sure (unless I'm doing something wrong -help?!) its still only looking for in modname/data first then rtw/data. I suppose that does at least mean BI vanilla folder should be able to remain untouched!

(Also recently realised that TFT mod Stuie mentioned having working via modname, is I think for RTW not BI unless there's a second version of it.)

It still could be worth getting this to work even if its not as neat as RTW mod -mod solution. Unfortunately I know no-where near enough to advise on music pack issues etc.