Does the current systemd journal not work for you today? On CoreOS, your app outputs to stdout which should log into docker which feeds logs into the journal which fleet exposes with fleetctl journal myapp.unit.

I realize that this isn't a proper logging product or anything, but it should be enough to hold you over. If this isn't working for you or you have a different use-case, hop in #coreos on Freenode and we can try to figure something out.