The documentation defines the following non-terminal:

User ::= '!'* username |
         '!'* '#'uid |
...

which implies that you can use the comment character to specify a user id
instead of a user name.

Later on in the documentation:

The pound sign ('#') is used to indicate a comment (unless it occurs in the
context of a user name and is followed by one or more digits, in which case it
is treated as a uid).

However, it looks like the lexer throws away any text from '#' to $.  Which
means the following is an invalid assignment according to visudo:

User_Alias FOO #500

FOO ALL=/bin/ls # error at this line for undefined User_Alias

I have made a local modification to fix this.  First, The user must escape the
comment character:

User_Alias FOO \#500

Then I have made a change in the parser (parse.yacc) in the WORD production, at
line 775:

  if (strcmp($1, user_name) == 0)
    $$ = TRUE;
+ else if ($1[0] == '#' && atoi($1 + 1) == user_uid)
+   $$ = TRUE;
  else
    $$ = -1;
  free($1);