I've managed to bind a custom model to an element at runtime. The code is here: http://jsfiddle.net/ZiglioNZ/tzD4T/457/
The interesting bit is that I apply the data-bind attribute to an element I didn't define:
var handle = slider.slider().find(".ui-slider-handle").first();
$(handle).attr("data-bind", "tooltip: viewModel.value");
ko.applyBindings(viewModel.value, $(handle)[0]);