Do the HVAC ducts need to hang away from floor joists

This will impact your shear resistance, but it significantly depends on how much of the joist is cut away and how much is resting on the brick. If the roof is so steep that the entire width of the joist is supported several inches deep, it shouldn't have any impact at all. But if it's so shallow that the joist is only an inch or so wide over the support, then of course it will fail. Unfortunately I don't have the specifics on how much support you need (I'm not a structural engineer) but it wouldn't hurt to get the opinion of an expert on this.

Just because you can't see the joists doesn't mean you cannot use your beam approach for exposed rafters. The structure is essentially the same, except for a thin layer of sheetrock between the two, which has little impact on the forces involved. The best thing to do is distribute any concentrated load over multiple connections. Use a flat 2x4, or better yet 2x6 that's long enough to span 4 joists, about 4 to 6 feet. Place the chair connection at the center using a large through eye bolt, using an over sized washer to spread the load into the wood. The nut and bolt end will end up punching into the ceiling, but that's OK. Holes are easy to patch if the beam is ever removed, otherwise, it doesn't show anyway.

Attach the 2x beam to each joist with two 1/4" dia. lag screws. Use a screw long enough to penetrate 2 inches into the joist material, probably 4 inches. Drill a 5/32 inch pilot hole before placing each lag to prevent splitting the joist. The resulting 8 screws is more than adequate to support any adult and will not significantly weaken the joists.