Given the way our packages work it’s tricky to work out how to build a single package work in isolation - we don’t have any explicit dependencies in our package files at present so creating the correct build environment for an out-of-context build is non-trivial.
The way I’ve generally gone about building packages is to use the
STAGE3_PACKAGES variables to develop each in turn. Stage 2 consists of things that are stable and stage 3 only the package I’m currently working on - effectively using the output of stage 2 as the build environment for stage 3 if that makes sense. This means that even though my new package is failing the iteration time is only that required to duplicate the existing root filesystem.
There may be a way to automate this to some extent giving something close to what you are suggesting but I need to think it through some more.