Need a userspace replacement for linux namespaces and bwrap.

Currently, flatpak and steam clients for ubuntu require namespaces to function properly. This limits the ability to run them on minimal/limited hosts such as docker containers. We require a seamless drop-in replacement that would mimic namespaces behavior without the corresponding kernel modules to circumvent this limitation. While we are aware that this has some serious security risk implications, maintaining some level of security guarantees that namespaces provide, to the extent that's applicable in this context, should be still possible on userspace level.
There are no requirements for programming languages or frameworks, but we suspect lower level solutions like c++ will offer better performance in this case. As such, applicants that can offer their experience with those will be given priority.