Much as with the GameCube, the MX "RTC-SRAM-ROM" chip sits on the EXI bus at address 0-1. http://hitmen.c02.at/files/yagcd/yagcd/chap10.html#sec10.3
The only data contained in ROM on the Wii is two fontsets (one Latin, one Japanese). They are compressed using a proprietary format (Yay0), and generally only used in GameCube mode.
Real-Time Clock (RTC) is 32-bit value, counting time intervals in seconds. To get the real time (seconds since January 1st, 2000 12am) add the counter bias saved in SRAM (or is it stored in NAND? [check])
The SRAM stores some miscellaneous settings that are only used in GameCube compatibility mode.