Lambics have been made in apple variations for years. One of the best known is Unibroue's Ephemere, which isn't really a lambic, but follows from the same concept/traditions. Two Brothers brewery made an apple beer for their holiday offering this year, it was called Avalon and quite good. I think the challenge will be retaining the apple flavor, since most of your juice is likely to be fermentable. I'd go with something closer to an amber ale, personally - the slight roasty/chocolatey notes of a brown ale would clash with the sweeter apple flavor/aroma.