Worth investigating: NEW Walabot DIY Stud Finder In-Wall Imager.
Also consider avoiding the metal studs and using one of the Wingits fasteners. These are considered acceptable for securing grab bars in drywall. Drywall is very strong if the pressure of a fastener can be spread out. Wingits do that, but I am not sure they are rated for the high static load.
At least one type of Wingits (RC-MAW35-2) requires a 3/4" dia hole in the drywall, and claim to work for walls up to 1.3" thick. Such large holes might be ruled to compromise the fire rating.