Yo dude, long time no see.
So far as I know, Scriptclip must produce same number of output frames as input. (+ same size, colorspace etc).
So 1 single pass solution is unlikely. Although could do it as realtime 1st pass, with auto second pass.
No idea what IOW means.
But see here:-
http://forum.doom9.org/showthread.php?t=172904
You might want to use YPlaneMinMaxDifference (Threshold=eg 0.2) to establish 'blank frame', and YPlaneMin or AverageLuma to establish level.