I like that scavenger idea odin suggested. It prevents ridiculous things like saccing ten bonwalls to it, but gives it synergy with... well, all PC. It would also synergize with things like earthquake (in-element, too!), and it still synergizes well with bone wall. (5 creatures attack into your wall, 5 walls destroyed) It would also get buffed from things like your dimshield/wings/sundials timing out, or saccing your own SoP/UGs. Also, a ruling would have to be made on whether replacing an old weapon/shield with a new one or flying a weapon would count towards the scavenger-for-permanents thing. Assuming memi even likes the idea, and I do; I think his/her idea would be even better with that.
The multiple times per turn thing as it is now does make me pretty nervous, given that you could scoop 7 bone walls along with 6-8 pillars at once for a rather nasty sized finishing blow. If it were limited to one per turn, I'd feel a lot better about it, but twice could be reasonable, too.