(Note that this happened before I started owning code pages, so I might be a bit confused about parts of this).
Support for HKSCS is now through Unicode 5. There's a download available, Microsoft Character Code Conversion Routines For HKSCS-2004, that has some routines to help applications convert HKSCS data encoded in Big 5 or the Unicode PUA to Unicode 4.1.
Previously we provided Hong Kong Supplementary Character Set (HKSCS) Support for Windows XP Platform which was effectively a replacement for the 950 code page with mappings to the Unicode PUA. Sometimes this was called code page 951 because of the name of the replacement file in the download. This is sort of a hack and doesn't work on Vista, you'll get a compatability warning if you try to install it, and if you force the issue I'm not gonna support whatever happens to your machine! :)
The safest bet for applications developers to have consistent support is to use Unicode, which supports the HKSCS characters now, so you don't have to worry about the PUA font mappings or other issues that have caused data portability issues between versions or systems.