That's basically what I meant. You could use those tube "spacers" used on the alt. They are thick and you can find them in a lot of different lengths. The through bolt would hold it in place and if the car is already painted, that would probably work. If it's a new build, I'd weld the ends of the tube to the frame.
Understood. The only thing I was thinking that would be slightly different is to actually have the tube run through the bracket (not just through the frame).
We do that sort of thing here at work (aerospace stuff), but the tube is precision OD and the match drilled hole is reamed so there is basically a line-to-line fit (very tight clearance fit). This prevents the fasteners from loosening. The fastener itself would take no shear load in this case. This is probably ridiculously overkill for a car... and it wouldn't be easy to do underneath the car.
Welding a tube into the frame would be a great solution as well, as long as you can grind the tube flush with the frame without losing all your weld.