Is there any way to change the opacity of the lister window so it's semi-transparent?
You'd need to use one of the many third party tools which do this.
If you do make the window translucent be aware that it might conflict with some viewer plugins (most likely ActiveX and Movie viewers).
I doubt it will cause anything worse then some graphical oddities or files not displaying but if you do see that then try turning off the translucency before any other troubleshooting, in case it's that.
You could try PowerMenu: abstractpath.com/powermenu/
It's a small application that does the job (and more) for free. I tried it with DOpus and it works, also with
the ActiveX viewers. As Leo wrote, it doesn't work with the Movie viewers (no crash, but a black viewer pane).
You could also try ac'tivaid, which is a script collection based on Autohotkey. It also provides free adjustable transparency values in the module "Windows Control", which you can use on almost any window by a hotkey.