3 possible issues:
1. Will local ordinances allow it?
2. Will the grey water "dirty up" the toilet? (black rings and such)
3. The system isn't as filtered obviously as the domestic system and some areas of the city have had issues with tiny snails clogging up sprinkler systems.
Of course my wife doesn't want me to do it and if I did I'd leave the existing plumbing in place to make an easy change over if needed.