pyqgis - Is it possible to change legend symbols in the QGIS print composer legend?

I have some layers with very complex data-defined symbols. If I use a legend in the print composer these symbols are not drawn correctly.

Is there a way to change the legend-symbols with pyqgis, so I can use custom PNG or SVG images instead of the default legend symbol for these layers?

I know how to add a Push Button to the Print composer and how to connect it with a function. I want to add a button to the Legend-Settings to allow me to replace the automatically generated legend icon with a custom image. So what I still need is the information of how I can access the legend-symbols with pyqgis/pyqt, and how to replace them with a QImage on a QLabel or something like that?

Very basic mockup of the button used to change the symbol:

Automatically generated Legend:

Legend with custom legend symbol:

I already found out how to access the items of the legend in the print composer but not yet how to access the symbol itself:

import qgis

from PyQt4.QtCore import *
from PyQt4.QtGui import *

activeComposer = iface.activeComposers()

for item in activeComposer:
    if item.composerWindow().windowTitle()=='test':
        for i in item.items():
            if isinstance(i,QgsComposerLegend):
                #print i

                #print i.model()
                legend = i
                for i in xrange(legend.modelV2().rowCount()):
                    posteleg=legend.modelV2().index(i, 0)
                    print posteleg.data()

Edit 1:

I have also found out how to Access QIcon-Objects of the legend-tree but cannot swap them yet:

def run(self):


        activeComposer = self.iface.activeComposers()
        #print(self.resolve('icon.png'))
        for item in activeComposer:
            if item.composerWindow().windowTitle()=='test':
                for i in item.items():
                    if isinstance(i,QgsComposerLegend):
                        legend = i

                        layerIcon = QIcon(os.path.join(os.path.dirname(__file__), "icon.png"))


                        for i in xrange(legend.modelV2().rowCount()):
                            posteleg=legend.modelV2().index(i, 0)
                            posteleg.model().iconGroup().swap(layerIcon)
                            print posteleg.data()

Here is one real life example where you can see a symbology combined of many symbol layers: This will end up in the legend like this:

As I need the proper symbol in the legend I would like to make a screenshot of my symbol, crop it and use it as an image in my legend.

I know I could just overlay a seperate image on top of my legend which covers the automatically generated symbol but I would like to have a "cleaner" solution that allows me to replace the symbols in the legend with custom images.

Edit 2:

In the meantime I have found another way to gain access to the legend entries:

from qgis.core import QgsLegendRenderer, QgsComposerLegendStyle

compDict = {}
for comp in iface.activeComposers():
    # workaround to get name: read it from window title
    compDict[comp.composerWindow().windowTitle()] = comp.composition()
if "mycomposername" in compDict:
    itemLlegend = compDict["mycomposername"].getComposerItemById("mylegend_id")
    if itemLlegend:
        print itemLlegend


tree_layer_layer =  itemLlegend.modelV2().rootGroup().children()
for item in tree_layer_layer:
        if item.layerName()=="MyLayername":
            print "match"
            QgsLegendRenderer.setNodeLegendStyle(item, QgsComposerLegendStyle.Group)

This allows me to access the QgsLayerTreeLayer objects and I can switch the legend Style (Group,Subgroup,Hidden). But I still have no idea how to access the legend-symbol. Any ideas?