I want to use a spare bathroom to grow edible hydroponic plants. The water supply under the toilet would be a good place to connect my plant sprouter. Is it safe to use this water since I'll be eating the plants?

The water supply to the toilet is the same as the water supply to your sink, so no problem.

Yes, it is safe. As stated in above answer, it is exactly the same water as in rest of your house, including kitchen sink and shower.