Since the additional amount is earmarked for the specific purpose of purchasing health insurance, the Department of Labor will likely consider the money to be a reimbursement, fringe benefit, or bonus, rather than part of the base salary (they have told us that line items don’t count toward salary). Therefore, I would not recommend counting it when calculating income for compliance with the overtime rule. The same would be true for other allowances, such as money to be used for transportation or cell phone service.
If you want to count the additional amount toward the FLSA salary threshold, I would suggest simply doing away with the earmarking. You could give the employees the same total compensation without indicating that a certain portion is for purchasing health insurance. The employee will receive the same amount of money, but you’ll avoid any kerfuffle with the Department of Labor. You are of course welcome to communicate to employees upon hire that the salary for the position has been determined with the cost of health insurance (or car or cell phone use) as a consideration.