Yes, it's possible, I've tried it. You have to set the "shaped" option in theme.xml. However, it seems that each pixel can only have a transparency of 0% or 100%, no in between, semi-transparency. However...I dunno if matchbox is handling this incorrectly or what, but when you maximize any window to fullscreen, the transparency is applied to the contents of the window where the titlebar *would be* on the screen. So, it works, just not good enough if you want to use fullscreen apps. Also, the transparency won't disappear immediately when you change themes, you have to switch themes a couple of times.

If you want to try it out, edit a theme's titlebar to have some fully transparent parts. Then go to your theme.xml file and change
to

Now that I think about it, it may be possible to change only individual layers of the titlebar frame instead of the entire frame. This might get rid of the errors. I'm going to try that out later.

Gaah, I uploaded just the titlebar but it was converted to jpg. Bah.