I contacted Patrick Volkerding to get is opinion on my project. Here is (part of) his answer:
"My outlook is that if I actually agreed with the concept of dependency tracking I'd likely just switch to RPM as the package system rather
than try to reinvent the wheel (along with all the wheel's problems)."
My opinion is slightly different of course, otherwise I would not have started this project, and I would like to just offer the possibility. If people out there creating TGZs agree, then maybe my efforts will not be all lost!
I have now got the dependency check working. I will improve the version comparison function so a depency on, let's say version 3.2, will result in a match for all versions 3.2.x.
The CVS version of installpkg mow allows the priority to be extracted from the package, if install/slack-priority is present.