1) You have wine and know how to use it (Running programs, winecfg, etc)
2) You have beryl running.
3) You have some amazing application that you want to run in fullscreen and in the background. (And possibly with different opacity, brightness, etc). I would recommend Dream Aquarium. thedarkmaskter has a tutorial on his blog on how to set it up. (Note, some setting will change from his instructions)
4) You have download the source code to Beryl Window Property Setter. It's a small application I wrote to get this to work. Should just download the file on the home page, then run make.
1) Run winecfg. Go to the graphics tab and check "Emulate a virtual desktop". Set "Desktop Size" to your desktop size. (I have mine set at 1400x900 and it runs smoothly). All other settings shouldn't make a difference. Although, I have "Allow the window manager to control the windows" unchecked. Hit OK.
2) Run the program through wine. ie: wine "C:\\Program Files\\Dream Aquarium\\Dream_Aquarium.scr". Now you should have the screensaver running in a window that is the size of your desktop. Make all settings (Number of fish, etc) you want now, because you won't be able to change them afterwards.
3) run bwpropset as follows: ./bwpropset -fs -s -sp -st -nf -ni -b -op .8
Then you'll see a crosshair. Select the wine window. If all goes well, you should have the window in the background of your desktop, much like xwinwrap.
Some more details:
-fs = fullscreen
-s = sticky
-sp = skip pager
-st = no focus
-ni = no input
-b = background
-op .8 = opacity at 80%
Play around with it until you find settings you like. Use the --help option for other features and possibilities.