I think it would be possible to borrow an idea from here Automated measurement of Z axis perpendicularity and jog the x/y in a circle around a single point so a single point on the work surface is used. Then you’re not measuring spindle relative to plane of surface (which may be poor) but you’re measuring spindle relative to plane of x/y movement.
In the end it’s probably equivalent to surfacing the spoilboard or shimming a true plane parallel to the x/y movement plane, but could skip a step and maybe more direct – fewer opportunities for errors to creep in.