i was asking a similar thing on this list a while ago.
Unfortunately the answer was that packages are specific to where they have been built.
You can't take a package built for one pkgsrc basepath and use them with another basepath.
As you found, the packages have some hardcoded paths in them. While you can override some paths, not all packages honor this and will fail on install and/or running.
So sadly in this case you have to use a build machine where you can build privileged on the same path where you plan to implant it to the target machine or directly rebuild your packages on the target machine.