Ability to Use Attributes within Information template to drive one of the chart of account segments

Summary:

We have a 9 segment chart of account. And our procurement accounting rules currently uses mapping between purchasing category and nominal account to derive the nominal account, rest of the segments are defaulted from employee expense account. However we now need to derive another segment on chart of account based on one of the fields on Information template ( which is a DFF on the information template configuration), however i don't find this as a source when creating a mapping set. Are there any options?

Alternatively if i enable a Context based DFF on Requisition Line, can we restrict the list of values within the LOV based on which purchasing category is selected? As we don't want the entire value set to be made available for selection ( i can create a mapping set in SLA to map Purchasing category to COA segment, but this will be one to many relationship i.e. One purchasing category mapped to multiple values ). Is this possible, if yes can someone provide a script/query for this value set definition.