1) I do not think that this is an issue at all as your slackbuild produces .txz packages in the /tmp folder.
Porteus is on hdd each slackware package in compressed form. They unfold at bootup and provide a live system that is modified thru a save change option:
You can 'run' Porteus from your hard drive whether it is an external or internal hard drive. We advise that you run Porteus from these media in its compressed form, commonly referred to as a 'frugal' install. We do not support decompressing Porteus to your system as a regular linux install. You should install Slackware instead.
Basic packages are slackware txz. They are built exacty the same way as they are for a normal slackware system. The only difference their compression into xzm (lzma2 method) and their activation at bootup or after bootup. Those xzm packages in the /porteus/modules package will be activated at bootup. See PORTEUS MODULES AN INTRODUCTION TO MODUDULES:
2) I have the impression that it is more a question of building correctly the prerequisite dependencies.
Do you have somewhere the final slackware packages for cups 1.7.0 and cups-filters? If you may?