During construction (HVAC installation) a properly sized return should've been placed near the ceiling in the open loft area IMO. This would not only allow your system in air conditioning mode to pull the heated air from the loft back down to the air handler and cool it off, but it would also allow you to circulate the heated air near the ceiling around to the entire home.
What to do after the fact? That all depends on whether there is access to install a dedicated and properly sized return ductwork and register to the loft ceiling area. This might take some inventive modifications and a bit of carpentry work to cover up the work done but it's the best option for the long term.
The ceiling fan(s) in the loft can be set to blow down during the summer, on continuously and the blower on your air conditioning system set to run continuously. This will blow the warm air back down to the main floor, mix it with the cooled air, and get it where hopefully the returns down there can return the mixed air and cool it off.
Heated air rises and requires work/energy to get it to go down.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.