The Gemara in Avodah Zara 35b says that one of the reasons why Gevinas Akum is assur is because they use "שרף של ערלה", or the sap of a tree that is assur because of orlah, as a rennet. This is noteworthy because Orlah only applies to the fruit of a tree (and the shomer l'pri, see Brachos 36b). It is permitted to use the parts of a tree that are not the fruit, and therefore if the sap of the tree of Assur because of orlah, it must be considered the fruit of the tree. Seemingly, it would have to be referring to a tree whose main product was the sap. This would be a proof that the bracha should be HaEitz. However, it is not clear if we indeed poskin like Tosfos in Brachos who states that all drinks receive a Shehakol, and therefore the Bracha would be Shehakol.
Edit: The Mishna in Orlah (1:7) cites Rebbi Eliezer's shita that the sap is considered the fruit of the tree. Rebbi Yehoshua (who the halacha is like) disagrees. The Tosfos Yom Tov says that Rebbi Yehoshua disagrees only when the tree produces other fruits, but if it does not, then the sap is considered the fruit. See the Tosfos Yom Tov for a discussion if the Rambam holds like this as well.
However, it is possible that if the maple tree produces no solid fruit it would have the status of an "אילן סרק" and the Shulchan Aruch poskins that the fruit of an אילן סרק receives the Bracha of Shehakol.