Seldon Island is just that, an island. I suppose you could ask someone with a canoe if they could give you a ride there from Gillette's castle boat launch (stash your bike in the woods I guess). Also, the six campsites are by reservation only in the summer, and the area is patrolled by boat by a ranger on the north end of the island. It's a fairly large island with alot of space between campsites, and no good trails between camps so you would have to know the name of your site, and have someone drop your there. The names of the sites are on signs visible from the water.
It's a beautiful island with rock cliffs, deep woods, marshes and wetlands and a natural spring. I think you can call Gillette's Castle to make a reservation. I think I can remember seeing canoeing groups camping right at Gillette's boat launch. Another park with camping in that area is Hurd State Park. No island but nice grassy campsites right on the river.