You can use this to change the names of your save files in RAM. On an emulator the changes you make won't be saved after you close the game, I haven't tested it on a real Nintendo 64 though.

 

Majora's Mask (U) version, at the File Select screen:
8040AF34 - Filenames, 8 bytes each. Not standard ASCII, characters that are not used are replaced with 0x3E, example:

Save name: DB

Value: 0x0D0B3E3E3E3E3E3E

Here is a list of possible values, found by yoshielectron (http://sites.google.com/site/jamesskingdom):
0x00 0
0x01 1
0x02 2
0x03 3
0x04 4
0x05 5
0x06 6
0x07 7
0x08 8
0x09 9
0x0A A
0x0B B
0X0C C
0X0D D
0X0E E
0X0F F
0X10 G
0X11 H
0X12 I
0X13 J
0X14 K
0X15 L
0X16 M
0X17 N
0X18 O
0X19 P
0X1A Q
0X1B R
0X1C S
0X1D T
0X1E U
0X1F V
0X20 W
0X21 X
0X22 Y
0X23 Z
0x24 a
0x25 b
0x26 c
0x27 d
0x28 e
0x29 f
0x2A g
0x2B h
0x2C i
0x2D j
0x2E k
0x2F l
0x30 m
0x31 n
0x32 o
0x33 p
0x34 q
0x35 r
0x36 s
0x37 t
0x38 u
0x39 v
0x3A w
0x3B x
0x3C y
0x3D z
0x3E <SPACE>

 

This should be the same for the European version (at a different memory address) but I don't know about the Japanese version because it uses more characters and three save files.