Adding a `userimg.jl` to base for custom build

I’ve started trying to use a user image to add default packages, make the REPL a bit more responsive after start and implement a few custom changes to base. I’m now also primarily using Julia 0.6 master, rebuilding more or less on a daily basis. Hence, I want to avoid building the system image twice and not use the build_sysimg.jl script.

The current setup is basically a local git branch that is rebased after each pull on top of master, including the build configuration and the mentioned additional customizations.

However, in doing so, I ran into a few fundamental issues in understanding how user images are supposed to work; and the manual is a bit short there.

What can and should go into a base/userimg.jl? Just using statements or also real code? Can I define new modules there or should these go into separate packages?
If separate packages, then were should I place those on disk, such that after building the sysimg the source files would still be found e.g. by @edit? Can I add additional precompile statements? And finally, how do I prevent Julia trying to subsequently precompile packages ('precompiling stale cache files`) that I already included in the user image?

I would assume that for the last points I should place those packages into some other directory than the default Pkg.dir()?

Thanks in advance for any advice!