We have considered it, however, because there's still a need for a firmware, the first stage bootloader and the firmware itself are using a common driver framework, it just makes things easier. Besides, the firmware will later run an RTOS (for example LittleKernel) and Uboot doesn't provide RTOS-like services.

U-boot already does support it: http://elinux.org/RPi_U-Boot

I think what is wanted is chainloading u-boot from the blobless firmware.