I did a Frank Lloyd Wright style years ago, only underground (underslab) system I ever did. Upstairs had slot diffusers paralleling the glass curtain walls. pretty cool house.
Since it was new I was able to run the four pvc flues for the furnaces behind the rock veneer on the central fireplace as they built it, and was concealed under the chimney cap on the roof.
also, some manufacturers allow the flue and intake to run through the crawls IF certain conditions are met, so the crawl wall could be a location for the venting w/o penetrating the roof. Warning- don't put under window- you'll see a cloud of steam whenever the furnace runs in winter (made the FLW house tricky with all those window walls).
4' is plenty of space to work in the crawl. I do like sealed crawls- since they're humidity controlled, you don't need to wrap the duct to prevent sweating, and there's no heat loss from the ducts to outdoors. Insulating the walls could result in less heat loss from house than if the floors and ducts were insulated.
Col 3:23
questions asked, answers received, ignorance abated