PS: I know my reply may not match your question exactly, it was mainly intended to give an overview of the current state. In short,
"preBuildCommands"is the closest thing to a custom target right now and the rest still needs to get carefully designed. CMake may be a good role model there.
No, that was exactly what I wanted to know, thanks!
If it weren't being considered then I might have started thinking about implementing and sending a pull request, but getting the design right is definitely very important.