With feed and return both at the top, the heated water will just whizz straight across the top rung and up the return without heating the rest of the rad.
Adding a long 'dip tube', like old single entry valves, reaching from the top almost to the bottom inside each side rail might work, but no guarantees - Plus you would have a problem of how to bleed air from the rad. when there's nowhere left to put a bleed valve.
A single small hole at the top of each internal dip tube, made with, say, a No. 60 drill may answer that one.
Just thinking out loud here - I've never tried it, and I don't know anybody that has, but it sounds like it might work....