I'd find it very useful if pkg_comp, still using sandboxctl and requiring root to run, could be configured to create an unprivileged user inside the sandbox and do an --unprivileged build as that user.
I'd use this in a VM configured to match the OS of a hosting provider where I have shell access but can't compile much of anything (thanks to process limits). I'd build binary packages in the VM, taking care that the sandbox $HOME matches my hosting provider $HOME, and then upload and install them.