I remember somewhere seeing where someone did that. Possibly on the y block site under this and that.
In the late 60’s, you’d see 4 point belts with the shoulder belt anchored behind the seat. Not to the roll ball or hard top, I think they finally decided the downward pressure if you needed them put too much pressure on the spinal column. I haven’t seen any like that in years. They all anchor higher now. My daily driver has Recaro seats made to accommodate roll bar mounted belts thru the headrest area.
Unless you’re pretty short, any anchor point in the bird will have the same effect even with a 3 point. If you find a solution let us know.