While your avoidance is understandable, given Broadcom's thoroughly disreputable history with regard to free software, it's not justified in this case. The Pi GPU is apparently fully enough documented that you can write your own software for it.
I could care less about the state of the GPU driver source and the typical complaints of RPi developers. Open, closed, whatever.
I'm more skeptical of a SoC architecture that has the CPU dependent on the GPU for its startup sequence. Or is this a matter of the GPU controlling the ARM's clock tree?
Most "bigger" SoCs have a secondary processor for SoC bringup/boot. (It's not uncommon to see a tiny ARM7 or something in this role). Since Broadcom succeeded in making a particularly general purpose core for their GPU (it seems that the GPU runs a full RTOS written in C), it only makes sense (well, to me at least) that it could take over that role as well to save on the transistor budget.
