managed to come up with this:

htmx.defineExtension('exclude-from-oob-swap', {
            onEvent: function (name, evt) {
                if (name === "htmx:oobBeforeSwap") {
                    var xhr = evt.detail.xhr;
                    var stop = xhr.getResponseHeader('hx-stop-oob') === "true";
                    if(stop){
                        evt.detail.shouldSwap = false;
                    }
                }
            }
});

but it stops the actual swap that should append the content, altogether with the oob swap ._.