Yo Matt, I'm interested in doing a cage too since I do track events a lot, but last time that I researched I though it can't be done in STX. I hope I'm wrong, I've actually started looking for an E30 to run spec and just leave the E36 for solo.
Also, I went to the National tour in SD and Pro Solo at ET and I don't remember seeing any STX car with cage, maybe it's just extra weight for SOLO but then the extra stiffness could help.
Not sure what happened last weekend but I've never really seen a crash happen to anyone that drives reasonably, its usually some over prepped or underprepped car in the hands of someone that doesn't understand drive/car limitation, like me when I flew off at the end of the banked turn into a ditch

Of course wheel to wheel is a lot different and more dangerous.
"Tell me and I forget. Teach me and I remember. Involve me and I learn." - Benjamin Franklin