TPLLookupField = class
   private
      FFieldName: string;
      FFieldValue: string;
   public
      property FieldName: string read FFieldName write FFieldName;
      property FieldValue: string read FFieldValue write FFieldValue;
   end;

   TPLBeforeLookupEvent = function(Sender: TObject;
                                   const ADataColumn: string;
                                   ALookupField: TPLLookupField): Boolean of object;

LLookupField := TPLLookupField.Create;
try
   Result := FOnBeforeLookup(Sender, DataColumn, LLookupField.FieldName, LLookupField.FieldValue);
finally
   LLookupField.Free;
end;