Comment 1:

Except for some logic around the Mode, should be as simple as (although untested):
func (h *Header) FileInfo() os.FileInfo {
   return headerFileInfo{h}
}
type tarHeaderFileInfo struct {
    th *Header
}
func (fi tarHeaderFileInfo) Size() int64        { return fi.th.Size }
func (fi tarHeaderFileInfo) IsDir() bool        { return fi.th.Mode == tar.TypeDir }
func (fi tarHeaderFileInfo) ModTime() time.Time { return fi.th.ModTime }
func (fi tarHeaderFileInfo) Mode() os.FileMode  { return os.ModePerm } // Not sure what
this should be
func (fi tarHeaderFileInfo) Sys() interface{}   { return fi.th }

Comment 2:

Please submit a CL! :-)

Comment 3:

Work has been done in the following CL: https://golang.org/cl/7305072/

Comment 4:

Owner changed to @adg.

Status changed to Started.

Comment 5:

This issue was closed by revision 44d38ae.

Status changed to Fixed.