Quick update... no progress . I was not able to make any sense of elf2boot files used to update Nokia devices, this was my primary attack vector.

I got another dev ARM board, with preloaded Linux on it, and I started to work with uBoot and understand how ARM Linux works in details at bootloader level. The problem with this approach is that I'd need to connect to JTAG port of my 5800 in order to get into the symbian bootloader, and somehow start uBoot from there - and I have very little electronics experience. Unfortunately the 5800 is my primary phone, so I am reluctant to mess with it.

I'll keep you posted if I get any progress on this.