For a patient receiving postmastectomy IMRT to chest wall with expander and regional nodes including IMN, what constraint would you use for dose to the contralateral chest wall/implant if the patient also had a mastectomy and reconstruction on that side?

I use similar numbers and tend to accept a more generous approach to achieve coverage if the patient has undergone a contralateral mastectomy vs intact breast.

ALARA, basically. The dose must be distributed somewhere, and the more you avoid the contralateral chest wall, the higher the dose the ipsilateral lung and potentially the heart may receive.