Good plan , threadless headsets don't care if there is a stack of spacers above the stem

just have the top spacer a little bit higher than the steerer tube so the adjusting bolt presses on the edge of the spacer , thru the loosened stem, to adjust the headset preload
I'd use a compression expander on the top rather than digging a star fangled nut into the Aluminum steerer,
if I were You.