If she won't come into milk without kidding again then you maybe want to look into tubes to squirt a long-acting AB directly into affected quarter. Maybe also worth discussing this with an experienced vet - I have no idea what the chances are of mastitis (re)developing in a dry udder or if any infection is "sealed" in-situ...
I am just discussing this with my vet as I have one of my BT's (a few weeks in-kid) still being milked atm, but down with mastitis and very little milk - so I am cutting down her concentrates (slowly), milk her out twice a day until milk is clear (it wasn't gunk in her udder, just thick and slightly non-white) and she has finished her AB's, then reduce milking as per usual until I think that's the last milking and then get vet to insert mammary tubes... well that's the plan.
Difficult situation all round....