Unless the trusses were designed with stone tile in mind, they are probably rated for L/480. To use natural stone, they would have needed to be specified at L/720, which is easy during the design stage, but may not be possible after the fact. Then, assuming they are in fact L/720, you need a second layer of ply, at least 3/8" thick, nailed with the ends offset and the joints not lined up. Then, a decoupling (could be a cement board or a decoupling membrane such as Ditra from www.schluter.com). Then the tile. this also assumes the spacing between the joists is within specs for stone tile. Deflection has two components - the subfloor and their support structure. Both must meet the L/720 specification. The second layer of ply is to isolate the joints at a truss or joist which act like mini-levers upon deflection - the second layer bridges them and negates the effect. You could almost certainly use a ceramic look-alike, but may not be able to use real stone. Check out www.johnbridge.com for further help on tiling.