Here's one version I've tried:
DetectHiddenWindows, On
#j::
IfWinExist, ahk_class MagUIClass
{
WinActivate, ahk_class MagUIClass
WinKill, ahk_class MagUIClass
return
}
When Magnifier is running and I type Win+J, the Magnifier window pops up, but it does not close. I'm running 64-bit Build 7100.