I have used Maple sap for a seasonal red ale. In fact I have one in fermenter currently. The way I approached the sap addition was to first plan out what I wanted my original of the beer to be, then add in all my grains/fermentables minus the sap contribution. So If you wanted a OG of 1.053, you would in your example have your gains/ferm. total 1.040 (1.053 OG - the 1.013 sap= 1.040). Then I would add Maple Sap to the Ingredients list in BeerSmith with the 1.013 sg and just keep adding a little at a time until the beers gravity went from 1.040 to 1.053. This way I had the maple sap included in the ingredients (you know you used all sap so the quantity wasn't as important, or accurate, as the contribution to the gravity of the beer and just having the reminder that sap was used to brew the beer for future use.
This is by no means an ideal way to add sap, but I found it to be easy for a beer I brew only once a year. By the way, my sap this year had a sg of 1.009. You have some good stuff.