This would be really cool, but is also a very tricky proposition. The LED backlights from
amBX require special device drivers. They work with most DirectX games to some extent, but require special plugins or profiles to work with specific games. Also, as far as I know, putting any game in full screen on one monitor is going to prevent any other applications from running full screen on a secondary or tertiary monitor. There has been some word that Windows 8 may allow separate full screen apps on each screen, but I haven't seen it myself.
Here's the closest thing I could come up with. Hopefully a more ingenious forum member will be able to suggest something better 'cause this is a really cool idea. This method isn't exactly what you're looking for, but it's the best I could throw together. It'll be based on the audio output of your games rather than their video output:
Keep your Eyefinity display group enabled and run your games that don't properly support triple-wide in windowed mode on your desired screen.
Then, download and install
Winamp. Watch out for the crappy toolbars and online services included by default in the installer. It'll give you the option to deselect them before installing.
After installing Winamp, you'll have access to the Milkdrop visualizer plugin by default. A lot of the Milkdrop visualizations are a little spastic for my tastes. There's plenty of other Winamp visualizers out there if Milkdrop's not to your liking. But, Milkdrop is good because it's provided by default and it allows desktop mode which will make your entire Eyefinity desktop show the visualization in the background.
Make sure that your Stereo Mix recording device in Windows is enabled. In Windows 7, right-click on the volume control icon in the system tray and select Recording Devices. If you see Stereo Mix with a green check-mark next to it, you're already in good shape. If you don't see Stereo Mix, right-click anywhere in the dialog box and make sure the Show Disabled and Show Disconnected boxes are checked. If you then see the Stereo Mix but it shows as disabled, right-click on it and click Enable.
Open Winamp. Press CTRL+L. Enter "linein://" as the URL and click Open. Click the Play button on Winamp. No audio should actually begin playing from Winamp, but Winamp should now be listening for the audio coming from your sound card. This will allow Winamp's visualizers to use your games' audio.
In Winamp, select the Milkdrop visualizer plugin with CTRL+K. You can also adjust the visualizer options from this screen by clicking the Configure button. Once you have selected the desired visualizer and configured it to your liking, click Start or click Close and then use CTRL+Shift+K to start the plugin. A window will appear with the visualization playing. Right-click on the visualization window and select Desktop Mode. The visualization should now span your entire desktop.
Run your desired game in a window. Play and enjoy the extra visuals! I'm going to look around and see if there's a Winamp plugin with more suitable visuals than Milkdrop, since almost all of Milkdrop's visuals are too distracting for gaming.