Here's some info on the Cannondale SI cranks:
Crankarms are constructed from a 2000-series aluminum that has twice the fatigue life of 7000-series aluminum.
Crankarms are mated to a thin-walled, hollow, oversized 30mm aluminum bottom bracket axle for superior balance of strength, weight and stiffness.
The two-piece, hollow crankarms are CNC-machined for optimum strength and light weight
So it looks like the crank arms themselves aren't forged, even though the stock that they are CNCed out of could be. If forged aluminum can indeed be welded, I could just use a chunk of forged aluminum cut and shaped down to the extension size needed to be welded on.
2024 series aluminum is unweldable, other 2xxx series are weldable. I guess I'd have to finagle the info out of Cannondale to be certain what they're made out of before I drop any cash if I decide to go this route.