Today I received Sharp CE-170TS serial cable. It was donated by one of OpenZaurus users which does not make any use of it. After some checking which RS232 port on my desktop machine is /dev/tts/0 and which /dev/tts/1 I got the second one working and installed minicom to get usable terminal (I used minicom years ago during studies when text terminals connected to multiport serial cards was something normal).
Few minutes later I got nice prompt in terminal:
OpenZaurus 3.5.4.1-rc1 c7x0 ttyS0
Now I can provide detailed kernel logs so maybe my 256M SD card will get more fixes from RMK :)