Backupmenu doesn't backup kernel. Kernel image in /boot/ is totally different thing - actual kernel used sits in special part of NAND, that isn't possible to being written via dd or tar -x. That's why You need flasher (be it on-device, or flasher3.5), to flash kernel.

Anyway, person with broken USB port, can easily re-estabilish connection via test pins beneath battery. Search forum for howto. One remainder - there was a FUD, that testpins omits protection circuits, and connect to chip directly. It isn't true - the only part that is not present - compared to regular U$SB port - is common mode choke. This, if used with poorly shielded cable and enviroment with high interferences, may lead to problems in getting full USB 2.0 speed, but certainly won't ''blow up'' Your chip.

/Estel