As I understand it, Sigma's policy on rechipping has been to offer it only on lenses which are in current production at the time of the request. I'm not aware they ever sold the chips to 3rd parties.
I'm not sure when Sigma dropped the 400/5.6 APO, but I think it was around 2003. At that time it could be rechipped to work with the Canon D30 and D60. Even if those chips were still available it's quite likely that they wouldn't be effective with more recent EOS models like the EOS 1Ds MkII.
You might want to look at the
EF 180/3.5L USM macro if you need a lens with high magnification (close focus) and long focal length for good working distance. It's a very good lens.