It's possible. Personally, I think it has something to do with auto-deteriorating relations. A CA programmer mentioned that in M2:TW the relations between an A.I. faction and the player automatically deteriorate over time, and that this goes faster at higher difficulty levels. So unless positive interactions take place, the A.I. will eventually hate the player. I think the same applies to R:TW, and it would explain why I didn't see any differences between diplomacy at H and VH difficulty. I always pay neighbouring factions a small, regular tribute, so the deterioration does not occur. Not everyone has reported success with my method, however (and the diplomatic A.I. remains stupid).

This is, BTW, not an area of the game-engine that is accessible to modders, so I am afraid there won't be any solution.