rasim from the Autoit forums was kind enough to answer this question of mine with a working example: http://www.autoitscript.com/forum/index.php?showtopic=86894&st=0&gopid=623824&#entry623824
#include <WinAPI.au3>
#include <WindowsConstants.au3>
#include <Misc.au3>
#include <ScreenCapture.au3>
Global Const $WM_LBUTTONDOWN = 0x0201
Global $hDLL = DllOpen("User32.dll")
Global $Drag = False
Global $aCoord_Start
Global $aCoord_End
Global $iLeft, $iTop, $iRight, $iBottom
Global $sCapture = False
HotKeySet("{Esc}", "_Exit")
Global $pStub_MouseProc = DllCallbackRegister ("_Mouse_Handler", "int", "int;ptr;ptr")
Global $hHookMouse = _WinAPI_SetWindowsHookEx($WH_MOUSE_LL, DllCallbackGetPtr($pStub_MouseProc), _WinAPI_GetModuleHandle(0), 0)
While 1
If $sCapture Then
$sCapture = False
_ScreenCapture_Capture(@ScriptDir & "\Capture.jpg", $iLeft, $iTop, $iRight, $iBottom)
EndIf
Sleep(100)
WEnd
Func _Mouse_Handler($nCode, $wParam, $lParam)
If $nCode < 0 Then Return _WinAPI_CallNextHookEx($hHookMouse, $nCode, $wParam, $lParam)
Switch $wParam
Case $WM_LBUTTONDOWN
$aCoord_Start = MouseGetPos()
Case $WM_MOUSEMOVE
If _IsPressed("01", $hDLL) Then $Drag = True
Case $WM_LBUTTONUP
$aCoord_End = MouseGetPos()
If $Drag = True Then
$Drag = False
If $aCoord_Start[0] < $aCoord_End[0] Then
$iLeft = $aCoord_Start[0]
$iRight = $aCoord_End[0]
Else
$iLeft = $aCoord_End[0]
$iRight = $aCoord_Start[0]
EndIf
If $aCoord_Start[1] < $aCoord_End[1] Then
$iTop = $aCoord_Start[1]
$iBottom = $aCoord_End[1]
Else
$iTop = $aCoord_End[1]
$iBottom = $aCoord_Start[1]
EndIf
$sCapture = True
Return 0
EndIf
EndSwitch
Return _WinAPI_CallNextHookEx($hHookMouse, $nCode, $wParam, $lParam)
EndFunc
Func _Exit()
DllCallbackFree($pStub_MouseProc)
_WinAPI_UnhookWindowsHookEx($hHookMouse)
DllClose($hDLL)
Exit
EndFunc