Before you invest in a C version, let's discuss whether this feature is desirable.  The proposed function implements a very simple and not very common calculation.  Note that even dateutil does not provide direct support for this: you are instructed to use relativedelta to add weeks to January 1st of the given year.

If we are going to add yet another way to construct dates, I would like to consider some universal solution.  For example, a "make_date" function that would behave similar to the timedelta constructor: take a large number of keyword arguments and return a date.  For example,

make_date(year=2000, isoweek=5, weekday=3)
make_date(year=2000, isoday=63)
etc.