JPEG2000ASCImageDict is documented in distiller parameters 7.0. To implement it we need TileWidth and TileHeight in the encoding filter interface. Ralph, please pass the bug to me after improving the filter interface.
Reassigning to owner, probably a low priority issue.
Bug still reproducible in Ghostscript 9.03
The actual parameter is JPEG2000ColorASCImageDict. This used to control the creation of JPEG2000 compressed colour images. As far as I'm aware the JPEG2000 compression method is the subject of a number of patents. While royalty-free licences are granted for *de*coders, the *en*coder cannot be created without paying licence fees. We do not implement JPEG2000 in the free release of pdfwrite. The commercial release, which uses the Luratech JPEG2000 codec does allow for this. However, given that no commercial customer has asked for this, and the encoder interface is still lacking the TileWidth and TileHeight, I'm closing this as 'wontfix'. If a customer ever asks for it we can do it then.