Then you are exploiting potentially flawed PRNG which has been done and is completely feasible. Bitaddress uses randomness from different sources which would make it harder as you'll have to replicate both the tracked mouse movement as well as the randomness that was generated when the user enters the page.

These attacks can only work if they are using predictable variables as an entropy source. If and only if you can find a pattern in that generation, then you can reduce the search space significantly. Under no circumstances should any wallet be generating using flawed PRNG.