It would be very nice to have hp2xx support im imagemagick. hp2xx can be found at http://www.gnu.org/software/hp2xx. Please see also bug #58091. Reproducible: Always
This is a drop-in runtime requirement of imagemagick, so with bug 58091 closed, you can simply emerge it. I don't think this is requiring any changes whatsoever to the imagemagick's ebuild itself. *** This bug has been marked as a duplicate of bug 58091 ***