Marie
Remember that even in attack you will have a number of ingredients to add fibre to your diet - you will be eating 1.5 tblsps of oatbran a day, and you can also add rhubarb.
The problem with the benefiber is that it is 100% carbohydrate. Most of it should pass through you with out being absorbed, but it may well make it much harder for your body to move into ketosis.
I understand why you would like to use this product, particularly with such a long attack period.
Would you consider trying to go without for at least the first stages of attack, to see how it goes? You can always keep some aside, for emergencies.
Once you reach Cruise , you may well find, if the diet suits you, that constipation is a thing of the past. Processed carbohydrates (white flour, white sugar etc) are one of many factors which can cause constipation.