Add data members to interface data structures?

[Max TenEyck Woodbury]

Would a patch that allows data members to be hacked into interface data
structures be acceptable?

Specifically, this would add sequences that follow this template:

#ifdef <iface>_IFACE_DATA
  <iface>_IFACE_DATA
#endif /* <iface>_IFACE_DATA defined */

for each inherited interface, in order.