I think it will depend on the supply of dross.if one site will produce enough to feed one reactor ,it would probably be on site.If there are several smaller sources in,say in a city or state city,then a central location may be the best approach,heat loss and transportation would diminish some of the benefits.
   The fence may be around the aluminum producers,but it's products lead to many more fences,anyone that make cast aluminum parts,produces dross.Tesla may need a couple for their castings