The greens don't actually need to stay completely separate from warm stuff - it's more about timing and temperature. I've done this plenty of times at home: toss your fresh lettuce with a light vinaigrette first, then add the roasted veggies once they've cooled to maybe body temperature or slightly warmer. The dressing creates a little barrier and the greens don't wilt as fast as you'd think, especially if you use heartier lettuces like romaine instead of something delicate like butterhead.
Another trick that works really well is just keeping the warm and cool components in separate bowls until the last second before eating, then doing a quick toss at the table. sounds fussy but it takes 10 seconds and nobody gets sad lettuce.