You're right, it seems that the image viewer for AC is an IE browser pane. If you have an activeX plugin for the file type you wish to view it will work. This is why it is possible to view Excel and Word documents right in the viewer. With the right browser OOo can be viewed in AC too.

With PDF X-change go edit>preferences then uncheck "show splash screen". Now, select "file associations" and check "display PDF in browser"