I took some more pictures and created a collage. Hopefully the collage is easier to view than multiple individual files.
Washers. Yes, they probably would do the trick. My only hesitation is that my hub becomes wider than my rear spacing. The hub is already sitting pretty tight. My rear spacing actually measures around 131mm to 132 mm when measured with my ruler. The hub seems to be 131mm exactly. Washers could bring this out to 133mm or even 134mm. It's worth a try and I will make a trip to Home Depot this weekend. For this experiment, I will simply put the washers on the axle ends rather than embed them into the axle. See the first attachment.
I pushed the hub all the way back into the dropouts. You can see that the 13t cog is pretty far from the chain stay but is awfully close to the seat stay. Drape a chain around the cog and you can see hub it will rub. Then I turned the dropout screws all the way in and that pushed the hub almost halfway down the dropout. The chain almost clears the seat stay but now the cog is really close to the chain stay. There's no chance of the teeth grinding into the frame but it does make you pause and think.
I suppose I can get longer drop out screws and push the hub even lower to the point where it would just get ready to hit the chain stay. With the quick release and drop out screws holding the hub in position, I will probably be OK. After all, if everything fails, the hub and wheel will go up into the drop out and the chain would be rubbing against the seat stay. However, I don't know how this will affect my tire clearance. All I have now is hub. I haven't built my wheel with it just yet.
DaveSSS is right in that the 13t cog is probably too big to be in that position. Could I build up my own hybrid mix cassette? Suppose I go 12-13-14-15-17-19-21-23-26-29? Is there a way to mix and match Campy cassette sprockets? Has anyone done it?
It may sound desperate but is there anything a competent frame builder can do to the frame to accommodate the cassette?
-- Boris