How can I make “ls” show dotfiles first while staying case-insensitive?

OP was very close with editing /usr/share/i18n/locales/iso14651_t1_common, but the trick is not to delete the line

<U002E> IGNORE;IGNORE;IGNORE;<U002E> # 47 .

but rather to modify it to

<U002E> <RES-1>;IGNORE;IGNORE;<U002E> # 47 .

Why this works

The IGNORE statements specify that the full stop (aka period, or character <U002E>) will be ignored when ordering words alphabetically. To make your dotfiles come first, change IGNORE to a collating symbol that comes before all other characters. Collating symbols are defined by lines like

collating-symbol <something-inside-angle-brackets>

and they are ordered by the appearance of the line

<something-inside-angle-brackets>

In my copy of iso14651_t1_common, the first-place collating symbol is <RES-1>, which appears on line 3458. If you file is different, use whichever collating symbol is ordered first.

Details about character ordering with LC_COLLATE

<U002E> has three IGNORE statements because letters can be compared multiple times in case of ties. To understand this, consider lowercase a and uppercase A (which are part of a group of characters that actually get compared four times):

<U0061> <a>;<BAS>;<MIN>;IGNORE # 198 a
<U0041> <a>;<BAS>;<CAP>;IGNORE # 517 A

Having multiple rounds of comparison allow files that start with "a" and "A" to be grouped together because both are compared as <a> during the first pass, with the next letter determining the ordering. If all of the following letters are the same (e.g. a.txt and A.txt), the third pass will put a.txt first because the collating symbol for lowercase letters <MIN> appears on line 3467, before the collating symbol for uppercase letters <CAP> (line 3488).

Implementing this change

If you want the period to come first every time a program orders letters using LC_COLLATE, you can modify iso14651_t1_common as described above and rebuild your locations file. But if you want to make this change only to ls and without root access, you can copy the original locale files to another directory before modifying them.

What I did

My default locale is en_US, so I copied en_US, iso14651_t1, and iso14651_t1_common to $HOME/path/to/new/locales. There I made the abovementioned change to iso14651_t1_common and renamed en_US to en_DOTFILE. Next I compiled the en_DOTFILE locale with

localedef -i en_DOTFILE -f UTF-8 -vc $HOME/path/to/new/locales/en_DOTFILE.UTF-8

To replace the default ls ordering, make a BASH script called ls:

#!/bin/bash
LOCPATH=$HOME/path/to/new/locales LANG=en_DOTFILE.UTF-8 ls "$@"

save it somewhere that appears before /usr/bin on your path, and make it executable with chmod +x ls.

You can use the shell's sort order instead (which may not involve the locale's collation order; bash, AT&T ksh, yash, tcsh and zsh give the expected results, mksh and dash don't. fish seems to give a case insensitive order but gives different results when there are non-ASCII characters):

ls -dUl -- .* *

This gives ls an explicit list of files (and directories) to list, and deactivates ls's sorting (-U, which is a GNU extension).

There are a few caveats, depending on the shell you're using.

• With zsh, the default nomatch option will cause the command to fail if the directory doesn't contain both hidden and non-hidden files; you could disable nomatch to avoid that, but better would be to do set -o cshnullglob instead (and the command to fail only if none of the globs match like in (t)csh or early Unix shells).
• With zsh, pdksh and its derivative and fish, .*'s expansion doesn't include . and .., so this matches ls -Al. With other shells . and .. are included so it matches ls -al. In the latter case you'd need to change the globbing patterns to exclude . and .. (ls -dUl -- ..?* .[!.]* *).
• Except in fish, (t)csh or zsh, if any of the globbing patterns don't match anything, ls will produce an error message; you can avoid this either by setting the nullglob option (in bash or zsh at least), or by redirecting stderr to /dev/null (ls -dUl -- ..?* .[!.]* * 2>/dev/null). If you use nullglob, watch out for the potentially-surprising behaviour that causes (see Shell eating `?` characters). fish behaves like bash with nomatch except that when interactive, a warning message will be issued for each glob that has no match.

(With thanks to Stéphane Chazelas for all the feedback!)

You might simply use two separate ls commands:

$ ls -dl ..?* .[^.]* 2>/dev/null ; ls -dl *
-rw-r--r--. 1 sparhawk sparhawk 0  8 Jun  09:29 .a
-rw-r--r--. 1 sparhawk sparhawk 0  8 Jun  09:29 .b
-rw-r--r--. 1 sparhawk sparhawk 0  8 Jun  09:29 a
-rw-r--r--. 1 sparhawk sparhawk 0  8 Jun  09:29 A
-rw-r--r--. 1 sparhawk sparhawk 0  8 Jun  09:29 b
-rw-r--r--. 1 sparhawk sparhawk 0  8 Jun  09:29 B
-rw-r--r--. 1 sparhawk sparhawk 0  8 Jun  09:29 你好嗎

Unlike the other answers so far, this approach displays the dot files first avoiding the . and .. entries, then the remaining entries in ls alphabetical order.

@StephenKitt answer's might be improved though to achieve the same result:

$ ls -dUl ..?* .[^.]* * 2>/dev/null