I would say that that is not a very good idea. Like people have said above me, it would look very rough and you cannot really sand it. when it comes to brittleness, grout is the best candidate. If you lay tile on a flexible surface, and you grout it up, in a while, the grout with fracture, chip, crack, etc.
and that is while making a square with relatively even-thickness for the grout lines. You'd have to spread this on incredibly thick, which isn't practical for sanding at all. Also, if you even managed to succeed in sanding and detailing, and all that good stuff that bondo is made for, one good hit, bump, drop, whatever, would easily break your finish.
If I were you, I would just swing by Wal-Mart and pick up a $19 gallon of bondo.
A stone master chief would be cool though!