The problem with welding the extrusions has to do with the heat treatment (tempering) process that the extrusions go through after they're extruded.
The process increases the tensile and yield strength of the material significantly - by a factor of at least two.
When you weld a crack, there's an area around the weld (the Heat Affected Zone - HAZ) where the tempering has been removed. It's not so much a brittle spot as a soft spot.
Read this more more info on the 6061-T6 aluminum that's used for the extrusions: https://en.wikipedia.org/wiki/6061_aluminium_alloy
That being said, a 5" vertical crack, a foot below the hounds (that the proper term for the standing rigging attachment point) could probably be successfully welded by an experienced welder who can control the heat to minimize the HAZ. You really have nothing to lose.
I'd certainly keep an eye on it afterward to make sure the crack doesn't reappear or some strange deformation doesn't develop.