It wouldn't be pretty, but I'd be tempted to repair it with carbon. Take the paint off around the seat tube/top tube junction, file the welds smooth, and then lay up plenty of UD carbon tape and epoxy, using vinyl tape with pinpricks in it as a compression layer. I think I'd try to get the tape going over the top tube, round the seat tube, then under the top tube in a sort of figure of 8 shape. Then sand the carbon smooth.
It's a bit of a bodge, and I'm sure the purists will be choking on their Richard Sachs branded mocchachino, but it'd work.
It wouldn't be pretty, but I'd be tempted to repair it with carbon. Take the paint off around the seat tube/top tube junction, file the welds smooth, and then lay up plenty of UD carbon tape and epoxy, using vinyl tape with pinpricks in it as a compression layer. I think I'd try to get the tape going over the top tube, round the seat tube, then under the top tube in a sort of figure of 8 shape. Then sand the carbon smooth.
It's a bit of a bodge, and I'm sure the purists will be choking on their Richard Sachs branded mocchachino, but it'd work.