You could lower the bottom/front bolt on the caliper a good 4 or 5, that would also bring it forward a bit. As a result, the caliper would both move forward as rotate anti-clockwise giving you a lot more room between the caliper and seat stay.
That does mean you need a redesigned disc tab.
Edit: I only just now got what you meant by "The caliper touches the disc brake tab". Never mind.
You could lower the bottom/front bolt on the caliper a good 4 or 5, that would also bring it forward a bit. As a result, the caliper would both move forward as rotate anti-clockwise giving you a lot more room between the caliper and seat stay.That does mean you need a redesigned disc tab.
Edit: I only just now got what you meant by "The caliper touches the disc brake tab". Never mind.