I am not familiar with the Tools system of BAAN LN. They may have better flexibility than BAAN IV c3.
HOWEVER, I would almost think that they could not have disturbed the foundation of the 'standard program' because when a type 3 session is generated (or any type for that matter), the generator EXPECTS the index to be defined at the DATABSE level - that is what allows us to choose how many fields of an index are the view fields and how many in the detail.
(As far as I have understood your requirement) In terms of how I would do this (maybe you have the code and license that will allow to do this):
- Session 1 - type 2 session - display project level data
- sub-session 2 - type 3 session - on selection of the project in session 1, zoom to the sub-session to display the details by BOQ ( assuming that this field is also part of the index at the database level - if not, consider possibility of adding it - but that will need some amount of impact analysis, so something else does not 'break')