Although I've just realised that might not be any different to just having the function in the neopixelbus.cpp file. @Makuna the ram attribute, u found it has to be in a c file to work no? All the functions in hardwareserial.cpp have this attr too, does this also mean that it doesn't work there either. Ps got an email from esspressif saying they could not reproduce the rebooting! Can I send u an email?
Their email was a bit weird. I wrote back with more clear instructions, they said they could not reproduce it with the 1.3SDK using arduino... which i find odd, as i sent them the sketch and well... any version of the SDK i tried after the one that doesn't crash, caused a crash.. will keep you posted!