TextPad is my primary text editor. As with most text editors, it has it's own primitive compare tool. While there's no way to specify an external compare tool in TextPad, I did manage to put together something that's pretty darned close.

I set up an external tool in TextPad, which is a CMD file and takes as a Parameter $File. The CMD file runs an AutoIt script.

TextPad uses CTRL-F9 to compare the two most recently selected files. I use ALT-F9. Select File 1. ALT-F9. Select File 2. ALT-F9.

Violets!

CMD file

Code:
"C:\Program Files (x86)\AutoIt3\AutoIt3.exe" "C:\Users\<username>\AppData\Roaming\Helios\TextPad\7\Tools\BeyondCompareQueue.au3" %1
AutoIt script

Code:
#include <File.au3>

Opt("MustDeclareVars", 1)	;0 = no, 1 = require pre-declare

Local $Quote = chr(34)
Local $InputFile = $CmdLine[1]

Local $ClipboardContents = ClipGet()
If FileExists($ClipboardContents) Then
	If $InputFile = $ClipboardContents Then
		Local $szDrive, $szDir, $szFName, $szExt
		Local $TestPath = _PathSplit($ClipboardContents, $szDrive, $szDir, $szFName, $szExt)
		MsgBox(4096, @ScriptName, "Attempt to compare " & $szFName & $szExt & " to itself")
	Else
;		Local $Command = $Quote & "C:\Program Files (x86)\Beyond Compare 3\BCompare.exe" & $Quote & " " & $Quote & $ClipboardContents & $Quote & " " & $Quote & $InputFile & $Quote
		Local $Command = $Quote & "C:\Program Files (x86)\Beyond Compare 4\BCompare.exe" & $Quote & " " & $Quote & $ClipboardContents & $Quote & " " & $Quote & $InputFile & $Quote
		Run($Command)
	EndIf
Else
	ClipPut($InputFile)
EndIf