This sounds very similar to my slirpnetstack, which is using gvisor netstack to do, which I call translating L3 (packets) into L7 (userspace syscalls like connect()):

https://github.com/majek/slirpnetstack/

(btw, gvisor netstack, while not without problems, is likely to be faster than libslirp, see benchmarks https://github.com/rootless-containers/rootlesskit/pull/101#... )