you're allowed to ride up to headley overlook. the highlands trail frequently uses multiuse trails to connect hiking only sections. the picture of a mtn biker in the official mahlon map/brochure is taken on headley overlook, heading for the singletrack, so i wouldn't worry too much about it. the no helmets bit is questionable, as there are some slightly rougher sections between there and weldon rd. on the rail bed, whatever.