Really rough Rule of Thumb is to guessimate the dynamic loading to be three times the static weight. So if the spare weighs 100 lbs sitting there it will "weigh" (roughly) 300 lbs. at the bottom of that G-out. Next consider that the CG of the spare is in the center of it, not at a sidewall, so there's some leverage involved because that weight acts at the CG - not where it's convenient.
Next think about how that 300 lbs. loading is not a one-time thing. 300 may be the max (& it could be more if you really badly misjudged something), but the spare is going to "weigh" almost that much over every bump. Then factor in that aluminum has a finite Fatigue Life. Mild steel has an infinite fatigue life, but typical Fatigue Life calcs done on aluminum parts & structures is for 500,000 cycles. That seems like a lot of cycles, but how many wash-board ridges have you driven over? Each one of those is a cycle. So are G-outs, potholes, etc., etc., etc.
Maybe by running thru all of the calcs the case can be made for hanging the spare off of the jack brackets, but educated intuition tells me it won't work for very long.