How can I attach a top plate for a wall that falls between joists?
You plan is sound.  You really only need blocking every four feet or so, and at each end. The rigidity in your top plate will keep things from moving if it's securely attached to the blocking. 

If, by chance, you have no insulation in the attic you could install blocking from below. Cut some 3" by 8" slots in the drywall inside the wall line for access, and insert the blocks through there. Install them either spanning the slot or adjacent to it.