DOS TSR for remapping caps lock to control (see comment at the end for pre-built binaries)

capsctrl.asm

; DOS caps lock -> ctrl remapper
; Author: John Tsiombikas <nuclear@member.fsf.org>
; This program is public domain. Do whatever you like with it
; build with: nasm -o capsctrl.com -f bin capsctrl.asm
	org 100h
	bits 16

	mov ax, 0900h
	mov dx, msg
	int 21h

	; get current keyboard interrupt handler
	mov ax, 3509h
	int 21h
	mov [orig_seg], es
	mov [orig_off], bx

	; set our own interrupt handler in its place
	mov ax, 2509h
	mov dx, kbintr
	int 21h

	mov ax, 0900h
	mov dx, msg_done
	int 21h

	; terminate and stay awesome
	mov dx, end_of_code
	int 27h

KB_INTR equ 09h
KB_PORT equ 60h
PIC1_CMD_PORT equ 20h
OCW2_EOI equ 20h

KBFLAGS0 equ 0x17
KBFLAGS1 equ 0x18

KBF0_CTRL equ 0x04
KBF1_LCTRL equ 0x01

SCAN_CAPS_PRESS equ 0x3a
SCAN_CAPS_RELEASE equ 0xba

kbintr:
	push ax
	in al, KB_PORT
	cmp al, SCAN_CAPS_PRESS
	jz .caps_press
	cmp al, SCAN_CAPS_RELEASE 
	jz .caps_release
	; otherwise jump to the original handler
	pop ax
	push word [cs:orig_seg]
	push word [cs:orig_off]
	retf

.caps_press:
	; set ctrl flags
	mov ax, es
	push word 0x40
	pop es
	or byte [es:KBFLAGS0], KBF0_CTRL
	or byte [es:KBFLAGS1], KBF1_LCTRL
	mov es, ax
	jmp .end

.caps_release:
	; clear ctrl flags
	mov ax, es
	push word 0x40
	pop es
	and byte [es:KBFLAGS0], ~KBF0_CTRL
	and byte [es:KBFLAGS1], ~KBF1_LCTRL
	mov es, ax

.end:	mov al, OCW2_EOI
	out PIC1_CMD_PORT, al
	pop ax
	iret

msg db 'Installing capslock -> ctrl mapper... $'
msg_done db 'done.',13,10,'$'
orig_seg dw 0
orig_off dw 0

end_of_code:

Update: I since generalized this hack, made it able to remap caps lock to any key (configurable with rebuild), added a utility for finding the proper scancode to use for configuring it, and made it a full project at: https://github.com/jtsiomb/capsmap

The release archive of capsmap includes pre-built binaries capsctrl.com and capsesc.com for remapping caps lock to ctrl or escape: https://github.com/jtsiomb/capsmap/releases/download/v1.0/capsmap.zip