You could set up two macros like these, bind something like alt-shift-3, alt-shift-4 to them.

Code:
/run ConsoleExec("Autointeract 1")
/p ON
Code:
/run ConsoleExec("Autointeract 0")
/p OFF
Then just use a regular HKN toggle to switch between them.
Code:
<Hotkey lctrl lalt lshift 2>
    <Toggle>
        <SendLabel w1, w2, w3, w4, w5>
             <Key lctrl lalt lshift 3>
    <Toggle>
        <SendLabel w1, w2, w3, w4, w5>
             <Key lctrl lalt lshift 4>

Or if you want a little button that stays around (this would use the same macros as above) you could use this. I called them aoe and st because I use those for toggling between my aoe and single target tanking.

Code:
// click to close button
<Command closeST>
	<TargetButton st>
		<HideWin>

// click to close button
<Command closeAOE>
	<TargetButton aoe>
		<HideWin>

// set up buttons at start and hide them
<Command AutoExec>
	<Createbutton aoe 50 30 175 30 "ON">
	<TargetButton aoe>
		<HideWin>
	<SetButtonCommand aoe closeAOE>
	<Createbutton st 50 30 175 30 "OFF">
	<TargetButton st>
		<HideWin>
	<SetButtonCommand st closeST>

// when you hit this hotkey it toggles which button is shown
<Hotkey lctrl lalt lshift 2>
    <Toggle>
        <SendLabel w1, w2, w3, w4, w5>
             <Key lctrl lalt lshift 3>
	<TargetButton aoe>
		<ShowWin>
		<AlwaysOnTop on>
    <Toggle>
        <SendLabel w1, w2, w3, w4, w5>
             <Key lctrl lalt lshift 4>
	<TargetButton st>
		<ShowWin>
		<AlwaysOnTop on>