Hi there,
For those who interested in customizing optiboot and to be able easy call functions located onto bootloader code I just wrote a tutorial from my new project about Ultra Low Power Wireless Arduino Node. This version include also into the bootloader a WS2812 RGB led driver callable from Arduino sketch .
There are 2 articles for the bootloading section.
the project presentation is here. The first part presenting why I customized the bootloader and the real implementation with technical explanations
All bootloading work is based on optiboot 6.2 which work like a charm, thanks to westfw for is great coding on original optiboot.
Do not hesitate to ask any question and use it for your own project.