The ST doesn't extend far enough above the TT to allow for a slide over clamp, so no grinding/filing down to do such. The carbon tubes suggest that any high temp solution like welding a bit back on will be fret with epoxy failure. But a welded bit is the most likely solution that has real merit and a very high cost. Besides the issue of keeping the heat located away from the epoxy is the question of the AL alloy and it's ability to be welded. I see aero space level skills...

Back in the day of Vitus and Alan bonded frames Harry H did many repairs on them. taking the junctions apart and replacing with new. For this frame I doubt that you'll find a donor "lug" without buying a complete frame. I don't think a year of pause will gather better answers. Andy