The GSA yoke is hollow so even if you TIG welded the bolt holes and recut the yoke, you probably don't have enough "meat" to carry rubber mounted endure bar clamps.
A custom mounting plate onto the existing bolt holes with mounting grommets made from 70 Duro Sorbothane sheet could give enough vibe isolation. BUT fixing the new bar clamps to the new plate so they avoid the mounting bolts and washers is an issue. I faffed about with templates but never got to cutting metal. See what you can come up with.
Fasstco Flexx bars are probably the best answer but you'll need a small mortgage. They also sell tuned mass dampers that will fit the GS bars.
I baulked at the costs because I have no way to test ride the products. The BMW with its horizontal vibes probably needs Flexx bars AND the tuned mass dampers to really sort the engine vibration. $360 for bars, plus $60 for inserts works out at £250 plus VAT and delivery.