I've not done just maple syrup--which wouldn't be a mead, BTW--but I've done any number of acermeads (syrup & honey). It used to be my "signature brew," and I made a batch every year up until recently. I went with a gallon each of honey and maple syrup, water to 5 gallons, and champagne-type yeast. Yeast nutrients optional. Age at least a year before drinking.
Just syrup should work, but I can't vouch as for the results. FWIW, spend the extra dollar$ and get real 100% maple syrup, no artificial flavors or colors or anything... In my experience, the artificial maple flavors won't go away or change during fermentation, and your end result will be--well, it'll still taste too sweet to drink, almost.
In accordance with the 2004 BJCP Style Guidelines, a mead made with maple syrup would be entered into the "Open CategoryMead" style (26C). Yes, you could ferment a maple syrup / water delution (you must delute the maple syrup or the yeast will not ferment it, as the sugar content is too high) but that would Not be mead.
I would be interested in the flavor of a "maple only" brew, but believe that the yeast used would have a large impact on the flavor. I would not say that the flavor would be "nasty". To see what you might get, try mixing maple syrup with some neutral sprits (Vodka, Everclear). The flavor and aroma of the maple should be present, but with more sweetness than you would have if you fermented the maple syrup to make the alcohol.
The real problem that I see, is getting good maple syrup at a reasonable cost. Here in the West, pure maple syrup is expensive. To make a 5 gal batch at an original gravity of 1.085 would require about 10.75 lbs or 3.5 quarts of syrup. That would be in the neighborhood of $35.00 for the syrup. That is what I pay for a 55 Lb sack of base malt! Perhaps a small 1 gal batch would be a good place to start if the initial flavor experiment tastes good?
I've heard talk of adding maple syrup to beer, but what about just maple syrup and yeast? Should work, right? Nasty?
12.5 lbs (1 gal)pure maple syrup
5 tsp nutr
2 packets Lalvin D47 yeast
5 Campden tables
water to fill carboy.
Brew it just like a mead. Add campden tables and let sit 24 hours before pitching the yeast. Make sure you let your yeast get going good at least an hour with a mixture of 2 cups of water to 1 cup of must. 2 weeks in primary cover with a towel then rack to secondary and attach airlock. Rack about ever 30 days. It usually will finish in about 2 or 3 months but it will slow to a crawl the last month. I never had to add finings as it cleared nicely and had it in the bottles at about 5 months.
I personally did not care for it as I don't like maple in very many things. The taste was dry and didn't have but hints of maple but the nose kept a strong maple quality. If I would ever do it again I would make it a bit sweeter. I made lots of good Christmas presents with them however.