what if I run a rear strut brace and drill and bolt into that?
I wouldn't.
Drilling the brace will weaken it. They are also designed to deal with a compressive/tensile force, not a bending one.
For the top mount I would use a section of thick flat bar (such as 65x6) drill a 1/2" clearance hole in it and weld a 1/2" UNF nut to it. Mount this under the parcel shelf (urethane is good for this) and mount the top harness to that using a 1/2"UNF grade 8 bolt (I assume the harness mount has a hole around this size). Make sure the bar is long enough to distribute any load. You may need to machine a spacer to allow for the distace between the parcel shelf panel and the trim above it.
Can you use the lower seatbelt mounts for the harness as well? If so, use longer bolts to allow for the extra metal on the harness mount.
If I recall, seatbelts use a standard imperial bolt (even in metric cars) so make sure you use ones with the correct thread diameter/pitch.