I've been searching for this cheat too, and I found this @ Gamespot:
Finally, there are many new cheats, including that if you rename a peep "M Brookes," you can place wall pieces and other scenery items on the edge of path tiles. This may not seem like a big deal, but believe me, the phrase "often requested" doesn't begin to do it justice! (For the curious, "Michael Brookes" is a member of the development team who has gone above and beyond the call of duty in his efforts to support the community.)
I haven't tested it yet, but I think it will work [:D]
*Edit* Just tested it, and it works! [:D]