"Would I ruin anything by tapping into the existing CHMSL wiring? I can't see anything in the FAQ that would indicate the brake light circuit must have a certain amount of resistance to disengage cruise or anything weird like that and the additional amp draw would be inconsequential"

Not sure about affecting cruise but it is almost guaranteed pop the brake failure warning light. The trailer wiring kit supplies supplemental voltage just for this reason,

Also keep in mind that the passive safety features of the Volvo are part of what keep us safe as well. Try not to limit any of our exceptional rear visibility )actually better that the 240).