The RKL uses a pretty simple I2C protocol. Each module has its own I2C address and command set. It's pretty simple to sniff out the address/command bits using the head unit and isolating each command / response cycle. You can use something like this to bridge between the head unit and the first power bar (or whatever module you want to sniff).
https://www.amazon.com/gp/product/B017B69D3U/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
It's a typical four wire connection on the jack (VCC, GND, SDA, SCL).
I had looked at this years ago and stopped at the point of controlling the PB4's and probe modules.
There is no reason you couldn't integrate this into reef-pi although it may be hard to find hardware now. I always thought DA should open source the hardware since their UI/UX was lacking and it was obvious they weren't going to put any real investment into furthering the product line. Which sucks because their hardware has always been rock solid. I still have a DA setup on a paludarium I did almost a decade ago now, and it's still going strong.