This is definitely the right way to do it.  I expect that I will have only minor nit-picks as I go through the patch.

1. You can probably just do

#define PyStructSequence_SET_ITEM PyTuple_SET_ITEM
#define PyStructSequence_GET_ITEM PyTuple_GET_ITEM

there is no need to repeat the argument lists.

2. I am comparing PyStructSequence_New and PyTuple_New:
 - PyStructSequence_New does not fill ob_item array with NULLs.
 - PyStructSequence_New does not call _PyObject_GC_TRACK

I believe tp_free gets inherited, so structseq tp_new should follow what tuple's tp_new does.  I am not 100% sure on the second point, though _PyObject_GC_TRACK may be redundant after PyObject_GC_NewVar. 

3. In structseq_repr, do you need PyTuple_GetItem?  I think you can get away with PyTuple_GET_ITEM, or better PyStructSequence_GET_ITEM.