How would you treat a large-cell neuroendocrine carcinoma of the breast with bulky axillary involvement with good response to neoadjuvant chemo followed by mastectomy/ALND?

I would favor comprehensive treatment as uncommon disease and has poor outcome and would err on side of over treatment

Agreed, I would treat comprehensively. In terms of dose, not much data but I would use standard doses unless concerns for gross or microscopic disease, though in this case margins are widely negative.