Topic: ZyXEL P-2602HWN-D7A, how-to build and exec OpenWRT image?

I started a wiki article for a ZyXEL router but I stuck with building an appropriate image and also I'm not sure how-to flash it correctly and execute it.

Here's my approach, correct me if I'm wrong:
1) Use openwrt-buildroot (12.09 final):
    Target System: TI AR7
    Target Profile: Annex-A DSL firmware
    Target Images: ramdisk, Compression (lzma)
2) This produces an image in bin/ar7/openwrt-ar7-initramfs.bin
3) I tried upload it into the RAM section (RAS Code part, see wiki) of the router with the command: ATUP9402c000,422694.
4) Using:
    ATGO(x)       run program at addr x or boot router
    to boot the image results in an error:

Exception occured!
EPC = 0x9402C000  
CAUSE = 0x00808028
STATUS = 0x10007C03
$RA= 0x9401DB68 

I somehow followed this example: … el:p_335wt

I'm not sure how to build a big endian image, because I guess it's necessary for this router?
Thank you in advance for any hint.