The Nest ones? I haven’t seen anything online of folks successfully flashing one. The first steps would probably be to solder on a USBC port and see what kind of access you can get over serial. There’s a picture of a Nest board (not the Pro) here, as well as info on what appears to be the correct usb connector. The OP also mentions that the Nest is lacking the developer button, but my guess would be that the function is still accessible by shorting the correct TPs. It doesn’t seem like that OP ever went through with the project, though, so maybe you’ll be the first!