There are no antennas on top of Marcy, nor will there ever be. Marcy is as environmentally fragile as they come. Besides there is no road access anwhere near there, only foot trails, also no electricity for miles. As far as Adirondack Mountians with antennas, look for fire towers as they pre-date the tightening of construction restrictions by the APA. Whiteface is accessable by a road constructed during the great depression and is home to the climate observatory, making whiteface the ONLY high peak with development at the top. As for who uses it, a quick geo search on the FCC would do the trick, but DEC and Forest are both up there, as well as a couple of ham repeaters. The ski jump towers are also popular locations for antennas.
Happy tower hunting!