Well, rkisp1 driver that drives ISP on RK3399 on Pinephone Pro is available upstream in fully supported manner for like 4 years. It also has full docs in publicly available Rockchip datasheets and partial docs in calibration manuals.

What have people done with it? Some mediocre support in libcamera? :) I don't think lack of handholding is an issue here.

Pinephone is a low volume product. What you need is to get the firmware for some common device that has shipped millions of units so that the number of people with the incentive to make it work is high enough to actually build a community.