I found this concept somewhat reminiscent of the Cooler Master case for the Framework 13 and worth mentioning here. Citronics are offering kits to repurpose the motherboard of the Fairphone 2 as an embedded microcontroller. The firm hopes to make kits for other brands and models of phone as demand grows, but for now Fairphone are easy to disassemble and well documented for hackers, so they start with that.
This concept might prove especially appropriate when the current Fairphone 5 comes to the end of its design life. Rather than using a mobile phone specific CPU, this handset adopted a more generalised SoC intended for internet connected devices. This choice ought to give more flexibility when used as the heart of a single board computer.
It is good to see more firms entering the market for “circular computing” and - whilst the Citronics web page seems to pitch towards industry rather than casual users - I look forward to hearing what hackers manage to do with them.