In the staging tree of @NeoRaider you will find a second stage bootloader for the wdr4900:
https://git.openwrt.org/?p=openwrt/staging/neoraider.git;a=shortlog;h=refs/heads/mpc85xx-spi-loader
Not sure why this did not find its way to upsteam OpenWrt. Maybe it still needs some work to finish it.