Hate to be the barrer of bad news but it's no BS.... clutch outer can cause the clutch to drag.
The clutch plate tangs ride on the inside of the clutch outer... and since the clutch outer is made of fairly soft aluminum it will form grooves where the tangs ride. These grooves, if deep enough, can cause the clutch plates to fail to release complete... causing clutch drag.
I used to see it all the time... especially on sport bikes and dirt/enduro bikes.
It can be called the clutch basket, clutch outer, outer housing... a rose by any other name
It's made of soft aluminum... the clutch rides inside the outer, the tangs on the clutch plates are harder than the aluminum of the outer. Yes it can happen on any GoldWing but because Wings are generally ridden easier than a dirt bike or a sport bike it's less likely to happen on a GL than on a CR or XR or a CBR.
How did the previous owners treat the bike? Remember that not everyone rides a GL like a touring bike and they might ride them more like a large sports bike. Strange as it might sound it happens. I just worked on a GL1800 that was purchased by a father for his son. He actually bought 2 06 White GL1800's. One for him and one for the son. The only difference is one(the Dad's) is a Nav unit and the other(for the son) is a std model. Anyway, the reason the bike was in my garage is I put a Traxxion rear shock and Traxxion fork springs in them both and installed AllBalls steering bearings in both bikes. Now, the son is grinding the footpegs...like I've never seen done before. This isn't just a mild touching of the peg... he is REALLY GRINDING the pegs!! He is actually also grinding the exhaust shield too!!!
After much discussion with Traxxion I sent the shock back to them for Dyno testing and evaluation. Well, the evaluation is that he's really a much more agressive rider than anyone suspected and that's the problem. Meaning that there isn't a problem with the bike... it's the ability of the rider.
So... although rare to wear a clutch outer on a GL it can happen. I'd investigate every other possiblity first before pulling the engine.