Not specific to SEA, but in the airports I've transited in the past year (which is admittedly fewer than any previous year), there are separate opt out and post WBI pat down lanes.

It would be my guess that if one opts out, and is standing in a line awaiting a rubbing, one is waiting in line with other opt outers.

Which brings up an interesting point. If there are lines, TSA is lying about 1 of 2 things (though likely both):

1) There are more opt outs than they claim (which is to be expected now that SDOO is now virtually impossible)

2) WBI with ATR has a high level of false positives. It makes me wonder what the level of false negatives is.