I my case, with wl500gp 128m ram mod.
1. Build a firmware from trunk
2. In build_dir manually change the build_dir/toolchain-mipsel_gcc-4.3.3+cs_uClibc-0.9.30.1/linux-188.8.131.52/arch/mips/bcm47xx/prom.c
3. Second build (this is soon fast)
4. Upload the firmware with tftp
5. First boot is OK (64MB RAM - with sdram_init=0x0009 as default, maybe from pin 9 ground restore).
6. I change nvram variables:
nvram set sdram_init=0x0011
nvram set sdram_ncdl=0
7. type reboot, and openwrt recognize 128m memory.. I can use reboot command and it works well, but I can't unplug the power cable, it would just brick and need a pin 9 ground restore.
by the way, I also can't get atheros card work for me at this box, as I mentioned here.