OK, I have got the packaging to work under MacOS X. The problem was that I set the destination to /usr/local/helios and put all relative path names into the pax.gz and .bom files. This was in an attempt to leave open the option to make fully relocatable packages, but this does not work out easily as you can not easily find out the installation directory of a base package if you have multiple add-on packages. I have now put root-relative path names (including the usr/local/helios prefix) into the .bom and .pax.gz files and set the destination to /. Now the packages install fine even if I re-install.