Took those photos.
From my research at the time found photos of subframes that were also cracking on the downtubes, didnt see anything to warrant the extra gusset you are talking about.
Yes the bikes filthy, but its a dirt bike and gets used as such. You can see I ran a gusset from the top mount of pillion peg bracket right back to the end of subframe. To run it that far back is overkill, but running it down will stop further cracking of the downtube. Didnt see any point in going any further as the OEM gusseting on the muffler side dont and mounts do a good job of reinforcing anyway.