We used a couple in the bottom of a multifuel stove to convert it to wood-burning only, but that's not going to use 30!
If you put them behind a boiler, they'll smooth out the heat radiated between 'on' and 'off' periods. When the boiler's hot, they'll absorb heat, reducing the amount that's radiated into the room, then when the boiler's cold, they'll radiate that heat back, possibly straight into the water in the boiler. I've been learning a lot about thermal mass, as our house has loads of it, but not very much insulation. I've come to the conclusion that thermal mass is not a whole lot of use when it comes to keeping warm (thick walls absorb heat then radiate it to the cooler side, or as a friend put it, "they just wick heat away.")
I wonder if the bricks would be good for building a pizza oven or something like that? From what I've just said above, I guess you'd need a more insulating material round the outside.
EDIT: Ah! I see OJ beat me too it, and with more confidence!