It appears that the MDFS user file format will require the user data to be loaded and sorted, if you plan to modify it in any way.
0 - 1 Entry number of first user entry beginning
with the letter 'A'.
2 - 3 Entry number of first user entry beginning
with the letters 'A' or 'B'.
4 - 5 Entry number of first user entry beginning
with the letters 'C' or 'D'.
...
28 - 29 Entry number of first user entry beginning
with a letter greater than 'Z'.
30 - 31 Entry number of the default user.
32 - 63 Not used.
64 - 127 First user record.
128 - 191 Second user record.
192 - 255 Third user record.
...
n Terminating user record (filled with &FF).
n + 64 URD/Lib names, pointed to by user entries),
maximum 80 characters, <CR> terminated.
Each user record is 64 bytes long, and is laid out in the following format:
0 - 9 User Identifier, terminated by <CR> if less
than 10 characters. Top bit must be masked out.
10 - 19 Password, terminated by <CR> if less than
10 characters. Top bit must be masked out.
The top bits of the User Identifier and the
Password hold a bit map of account ownership
to mid-range accounts:
Byte 0, bit 0 - Accounts &100 to &13F
...
Byte 19, bit 7 - Accounts &5C0 to &5FF
20 Boot option.
21 Flags:
Bit 0 - Password unlocked
Bit 1 - System privileged
Bit 2 - No short SAVEs
Bit 3 - Permanent ENABLE
Bit 4 - Acorn style library
Bit 5 - Run-only user
Bit 6 - Can't change auxiliary account
22 - 24 Offset from start of file to user's URD.
Set to 0 if URD is normal.
25 - 27 Offset from start of file to user's library.
Set to 0 if LIB is normal.
28 - 29 Personal account number (0 = no personal
account).
30 Bit map of high numbered accounts:
Bit 0 - &600 to &63F
Bit 1 - &640 to &67F
Bit 2 - &680 to &6BF
Bit 3 - &6C0 to &6FF
Bit 4 - &700 to &73F
Bit 5 - &740 to &77F
Bit 6 - &780 to &7BF
Bit 7 - &7C0 to &7FF
31 Reserved
32 - 63 Bit map of account ownership of low
numbered accounts:
Byte 0, bit 0 - Account 0
...
Byte 31, bit 7 - Account 255