For Both OS, I provides two scripts, one is for random themes(It must be different from last theme, so make sure you have more than one theme), and the other is for cycling themes in alphabetical order. I don't have a linux so I can't write scripts for it.
1. Setup Chameleon and put at least two themes into Chameleon's Extra/Themes folder. Then backup your boot.plist!!!
In Mac OS X:
Add random_themes.app or cycling_themes.app to your account's Login Items. It will assume your Themes and boot.plist in /Extra folder. If not, change the script accordingly.
1)You will need softwares such as MacDrive 8.0 to gain access to the HFS+ partition containing Chameleon.
2)Then you have to edit the beginning of the scripts (OSX_Disk_Label = "G:" ), change G: to your MacDrive disk label that containing Chameleon's Extra folder. At last, add random_themes.vbs or cycling_themes.vbs (or one of their shortcuts) to start->programs->startup.
3)I also made the scripts delayed for 20 seconds after boot, to wait for the loading of MacDrive. Change the value if you have problem running the scripts.
3. Enjoy different boot screens at each boot!
1. OS X and Windows have different alphabetical orders for file/folder listing. If you choose cycling themes in both win and osx, and you have both uppercase and lowercase theme names, it will not working properly. A simple workaround is making them all lowercase/uppercase, or add numbers in front of theme names to customize the cycling order.
2. Didn't do enough error checks, so make sure the paths of Themes and boot.plist are correct in the scripts, and the boot.plist must contain two lines: "<key>Theme</key>" and "<string>xxxx</string>" before use. And backup your boot.plist first!!!