Though I'd have thought that because maple syrup has an added sugar content, that probably comes close to a watery honey, I'd have thought that it's a bit hit and miss.
I doubt whether a normal hydrometer would actually have graduations high enough to measure it.....
I understand that it's normally a very specialist hydrometer or a refractometer that's specially graduated to measure sugar content of honey, syrups, etc etc...
Have a bit of a google about it, I'm pretty sure it will give you enough advice about the numbers, apart from that, I'd start by reducing down the maple sap gently, then add some sugar to taste. You can always boil it down a bit further the get the consistency you want......
12 gallons of sap = 1-gallon must. Honestly, though, it depends on what you're trying to do. I shoot for an OG of between 1.115 and 1.130. Here's a recent thread:
I think if I remember maple syurp is 66% sugar as required by law here in Vermont. Should be able to google it.. I would say decide what gravity you want and just shoot for that