Since the boot loaders are totally different between XP and the newer versions of Windows you wouldn't be able to add an entry in the XP boot options for 7. With the EasyBCD program you can still decide which will set as the default OS however if you elect to use a 3rd party program for this.
What gregrocker is pointing out about the boot device menu being a one time session boot is once you restart the system the default boot device/drive will load. The post screen will generally show which F key like F2, F4, F8, F11, or F12 key has been assigned for this. That depends on which bios is used for the make and model main board you have. Award, Phoenix, AMI, or other.
The one time boot option is also preferred when going to boot live from any disk such as going to install Windows or booting from any other live cd/dvd. Once the setup files are copied over to the destination drive during a Windows or other OS install the designated default boot device will load upon the restart. This saves unnecessary repeat trips into the bios setup.
One other thing about the XP boot.ini file or boot configuration file there is that it will open for viewing with NotePad when double clicking on it. You will need to right click on it and uncheck the read only box if you need to make any changes there and then when going to save the changes while in NotePad select the "save as" as "all files" not "txt" the default to see it overwrite the copy you have on the 7 drive.
Most will prefer the gui option with EasyBCD will some here simply prefer to select the other then 7 drive or device from the boot device menu at post time. The EasyBCD program does however include the option to restore the 7 mbr in the event you have any accident as well as setting the display time for selecting which version to boot into and setting which will be the default OS.