The solution is more simple than we think. Suppose we have a roof and a wall, both generated by Grasshopper. Even if the Archicad entities are locked by Grasshopper, it is possible to link them (in Archicad) using the 'Trim wall to roof' command. So the wall is trimmed, and it remains so, changing its shape accordingly, when we alter the roof parameters (for example the pitch).
The only disatvantage is that this interactions are not visible in Rhino.
Nic