Any herbicide that will kill violets needs to be absorbed by the plant and translocated to its root system in order to kill it. There is nothing that would be readily absorbed by the dormant roots. Violets are difficult to eradicate anyway. It usually takes repeat applications of a combination herbicide with 2,4-D and MCPP, and of course, you'd want to keep your rabbits off the lawn after you applied these herbicides. The other option is to physically dig out the violets, which sounds as though it would be an insurmountable task in your situation.