What I can do and you can't

I put this in /usr/lib/gstreamer-0.8/python, run gst-register and have a new element available that does what it says.

import gst
import gobject

 class ChannelSwap (gst.Element):
  __gsttemplates__ = (
      gst.PadTemplate ("src", gst.PAD_SRC, gst.PAD_ALWAYS,
        gst.Caps ("audio/x-raw-int, width=16, depth=16, channels=2")),
      gst.PadTemplate ("sink", gst.PAD_SINK, gst.PAD_ALWAYS,
        gst.Caps ("audio/x-raw-int, width=16, depth=16, channels=2"))
  )
  __gstdetails__ = ("Channel swapper", "Audio/Filter", "swap left and right channel",
      "Benjamin Otte <otte@gnome.org>")

   def __init__ (self):
    self.__gobject_init__()
    self.srcpad = gst.Pad (self.__gsttemplates__[0])
    self.srcpad.set_link_function (self.link)
    self.srcpad.set_getcaps_function (self.getcaps)
    self.add_pad (self.srcpad)
    self.sinkpad = gst.Pad (self.__gsttemplates__[1])
    self.sinkpad.set_chain_function (self.chain)
    self.sinkpad.set_link_function (self.link)
    self.sinkpad.set_getcaps_function (self.getcaps)
    self.add_pad (self.sinkpad)

   def otherpad (self, pad):
    if self.sinkpad == pad:
      return self.srcpad
    else:
      return self.sinkpad

   def link (self, pad, caps):
    return self.otherpad(pad).try_set_caps (caps)

   def getcaps (self, pad):
    return self.otherpad(pad).get_allowed_caps ()

   def chain (self, pad, data):
    data = data.copy_on_write ()
    for i in range (0, len (data), 4):
      tmp = data[i:i+2]
      data[i:i+2] = data[i+2:i+4]
      data[i+2:i+4] = tmp
    self.srcpad.push (data)

 gobject.type_register (ChannelSwap)
gst.element_register (ChannelSwap, "channelswap")

And no, it's not performant. Now to write the mail with the (quite big) patch to the list, so you can do that, too. You can thank jdahlin for making me start hacking on Python btw.