I think this should be rejected.  The OP's premise was that t.timetuple()[7] was unreadable, but in the modern python, the same can be written as t.timetuple().tm_yday.  The later is only slightly less readable than the proposed t.yday().

For the other half of the proposal, Marc-Andre's mxDate code translates into only slightly more complicated stdlib datetime code:

def date_fromyday(year, yday):
   return date(year, 1, 1) + timedelta(yday - 1)