I was thinking about this - a convolution filter with a long enough window would have a pretty good chance of identifying the right refrain location.

You're right about echoes, but it doesn't need to be perfect - the echoes are already probably below the detection floor for the takedown filter.

Also: If you identify and match up with a refrain, you'd still nullify that refrain. You'd just have to keep on your toes and move to the right location once you realized there was a mismatch. This is a classic cat/mouse "warring filters" problem!