I'm currently working on an XPM GUI extension that customizes the component library to insert content via XPM
I've found out that this list of components gets rendered by an XSLT stylesheet called DrillDownlistTemplate.xslt that can be found here : [tridion-install-dir]\web\WebUI\Core\Controls\DrilldownList\
Currently, i've almost achieved what I want to do by editing the DrillDownlistTemplate.xslt file itself, which is off course not the correct way to do it, as it's not supported to change any core tridion files.
I'm wondering if there is a way to override this xslt file (replace it by our own) to do some custom rendering.