I don't think it is possible to alter the pipeline created by playbin, it has internal code to auto manage this pipeline and manually modifying it will lead to unexpected results. You can update it using the given properties and signals, though.
You can use gst_bin_iterate_elements
or gst_bin_iterate_recurse
to iterate over elements of the pipeline to print them. It is also possible to use http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/gstreamer-GstInfo.html#GST-DEBUG-BIN-TO-DOT-FILE:CAPS to have a .dot file created. The dot file is a graph representation of the pipeline and can be converted into an image using the dot application. This way you have the full pipeline drawn into an easily understandable image.
It is hard to give you further advice as I don't know what you are trying to do by altering playbin2's pipeline, you can try looking at lower level elements, like uridecodebin or decodebin2, and look at the autoplugging signals to control what is automatically added by those elements. IIRC this can be also done from playbin2's level.
It seems that you are still using gstreamer 0.10, it is no longer developed. If you have no reasons to stick to 0.10, please move on to 1.0