This is the precedence order (in r640) for loading the boot.plist in the first scan before the booter starts:
"/Library/Preferences/SystemConfiguration/com.apple.lre.Boot.plist",
"/Extra/com.apple.Boot.plist",
"bt(0,0)/Extra/com.apple.Boot.plist",
"/Library/Preferences/SystemConfiguration/com.apple.Boot.plist",
"/com.apple.boot.P/Library/Preferences/SystemConfiguration/com.apple.Boot.plist",
"/com.apple.boot.R/Library/Preferences/SystemConfiguration/com.apple.Boot.plist",
"/com.apple.boot.S/Library/Preferences/SystemConfiguration/com.apple.Boot.plist"