The chain loading of *nix based systems over Windows 7 boot environment is like this:
1. Create an entry in BCD of type BootSectorLoader.
2. Give it a file to be loaded which has the contents of the MBR(master) or PBR(partition) boot record of the installed *nix system.
On my system I am very happy with chainloading the file boot.img from grub2 environment.
In Windows 7 you can use Ext2fsd version 0.50 to gain access to ext3,4 file system.
If the *nix loader is grub legacy you can first repair the *nix boot environment.
Then copy the MBR and PBR to two separate files with dd utility.
Examine with a hex editor if the last two bytes of the PBR (512 bytes long) are 55AA.
If not use the saved MBR-file for chainloading. Check here: Visual BCD Editor - How To
, Section 8. Create new Boot-Sector Loader (Linux, Mac OS X) If you destroy Windows MBR you have to repair it using the Windows Recovery CD or installation DVD - Startup repair or "bootrec /fixmbr" from command prompt