"Can anyone come up with a plausible etymology?"

I checked my unabridged dictionary, Paul, but no luck. It said that scouse was short for "lobscouse," or "lobscourse"* but it had no history for it.

It is intriguing that lobscouse is related to loblolly, and loblolly is, among other things, a thick gruel.

I suspect these terms are onomatopeic in origin - members of the lob, blob, glob class of sounds.
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Wait a minute! I've got it! Lobscourse was the original - thus we have the salad course, the fish course and the lobscourse. How do you want your (g)lobs cooked?