Good morning.
Well, I have now 2 feasible solutions...
5. Append the changed resources at the end of the vol.* files, and just update the corresponding *dir file.
6. After I thought about '5', I've returned to option '2' - compile locally in WinAGI, and use the binary diff - and checked it again - and I was probably wrong in my previous post... I've rechecked it on clean copy of the game, and indeed, it does exactly what expected. Most of vol.* files were the same, there was addition at the end of one of the vol.* files, and small change in the corresponding *dir file.
So, I've probably clicked 'Rebuild VOL files' in my previous attempt.
So, bottom line, problem solved.