I also heard Dr Feist's expanding gypsum tip at the Oxford conference and tried it on a barn conversion, where existing oak tie beams passed through the new t/f wall we were lining it with. The gaps b/w the tie beam and the i-beam studs were about 300mm longx10mm wide. It seemed to give a tight joint across the 10mm, but shrank and cracked along the 300mm dimension. I think it has potential as a solution but needs a trial on your job. Other problems are that gypsum is very hygroscopic, and ours were exposed to the 'summer' for a few weeks, became very damp and didn't seem to dry out when we had the roof cover on, presumably because of the small surface area and deep poured joint. That wouldn't happen on the top of a wall though as long as you replaced the roof above straight away. Depending on the detail, if you can do a simple shuttering and pour say 15-20mm thick, it could be a cheap quick not too fiddly solution. On a 300mm thick wall, I think you might get about 4 lin m per bag at 18mm deep??