Gouwa suggested I post this here as the answer might be useful to others too.
The RK3399 GPIO/I2C/SPI run in several power domains, some of which are configurable. Lines such as GPIO4_Cn and GPIO4_Dn run in the APIO4 domain which the RK3399 apparently supports as either 1.8V or 3.0V.
These are documented on the Edge v13 schematic as 3.0V, but can I configure the PMIC to run APIO4 in 1.8V mode instead without breaking anything on the Edge, i.e. set VLDO8 at 1.8V to run APIO4_VDD at 1.8V (instead of 3.0V), and VLDO6 at 1.8V to set the bias APIO4_VDDPST to 1.8V (instead of 1.5V) as in the datasheet, effectively making these 1.8V GPIOs?
I know some of these 3.0V lines are used for control purposes, e.g. HDMI_CEC, USB_PWR_EN, CHG1_CC_INT_L, etc., but as far as I can see, all of these places will work equally well with 1.8V vs 3.0V logic levels, e.g. the HDMI_CEC is level-shifted to 3.3V anyway, the SGM2551A EN-pin supports high levels as low as 1.4V. (I know this would also change the DBG_UART pin voltage from 3.0V to 1.8V, but that’s fine in my application.)
If it won’t cause problems, is it as simple as changing the relevant microvolt levels in the RK808-D setup in the device tree? I’d test this myself but can’t do so until the boards ship, so thought I’d ask the question instead!