Detailed design for the adapter is here: https://www.chromium.org/chromium-os/dingdong
If you feel like it, please file a bug.
Anyway, yeah, I've actually had that thought in the back of my head. I've actually reloaded this laptop 3 times due to the exact situation you mention. (The HDD is too small to leave CrOS on there).
It's been several months since I've let CrOS do it's thing and reflash the firmware... I should give it a shot.
Many moths ago, I was reinstalling CrOS it to try to get it to reflash the adapter so that MST on DP1.2 would work. But it never worked and furthermore, the Dell monitors I have stop working when you enable DP1.2 so all around... it was disappointing.
I wanted my Pixel to be my sole development machine... but when I'm sitting at my desk... I expect dual monitors... So I continue to sync my git repos between 3+ computers at a time. Woohoo.
Whoever wants to make a laptop with 16GB RAM, HiDPI screen (and not the shitty Lenovo panels), and 2x mini-DP out.... boy, I'd sure love to see it. (And the Surface Pro 4 actually falls in that category, but is not Linux friendly)
edit: Checked the dingdong history... not seeing too much relevant but I might give it a shot when I have a slow weekend. Thanks again!