Oops, I looked at the Python code again, and it seems to be sorting the strings by their length, not reversing the sort order like I did. Sorry, I didn't look at it very carefully earlier. Anyway, the easiest thing to do is probably write a wrapper type around StringSlice using Go's anonymous struct syntax. You only need to implement LessThan, since StringSlice already implements the other two.

  type StringSliceByLength struct {
      sort.StringSlice;
  }
  func (s StringSliceByLength) LessThan(i, j int) bool {
      return len(s.StringSlice[i]) < len(s.StringSlice[j]);
  }
  Sort.sort(StringSliceByLength(vs))

You can't put methods on a type from a different package (or in this case a builtin type).

Instead of methods, you could of course write normal functions taking []string, but only methods can fulfil interfaces.

This is the idiomatic way. It's actually the only way.