I don't know if there's a Mac equivalent, but if there are any Windows users interested in this, you can use AutoHotkey - just drop this in your .ahk file:
/*
Hotkeys:
Alt-W: make window less transparent
Alt-S: make window more transparent
Alt-S: reset
http://www.autohotkey.com/forum/topic57583.html*/
!w::
WinGet, currentWindow, ID, A
if not (%currentWindow%)
{
%currentWindow% := 255
}
if (%currentWindow% != 255)
{
%currentWindow% += 5
WinSet, Transparent, % %currentWindow%, ahk_id %currentWindow%
}
SplashImage,,w100 x0 y0 b fs12, % %currentWindow%
SetTimer, TurnOffSI, 1000, On
Return
!s::
SplashImage, Off
WinGet, currentWindow, ID, A
if not (%currentWindow%)
{
%currentWindow% := 255
}
if (%currentWindow% != 5)
{
%currentWindow% -= 5
WinSet, Transparent, % %currentWindow%, ahk_id %currentWindow%
}
SplashImage,, w100 x0 y0 b fs12, % %currentWindow%
SetTimer, TurnOffSI, 1000, On
Return
TurnOffSI:
SplashImage, off
SetTimer, TurnOffSI, 1000, Off
return
!x:: ; Reset Transparency Settings
WinSet, Transparent, 255, A
WinSet, Transparent, OFF, A
return