http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_778/29CFR778.418.htm
This is the section of the FLSA you need to be concerned with.
"(a) Under section 7(g)(1), an employee who is paid on the basis of a piece rate for the work performed during nonovertime hours may agree with his employer in advance of the performance of the work that he shall be paid at a rate not less than one and one-half times this piece rate for each piece produced during the overtime hours. No additional overtime pay will be due under the Act provided that the general conditions discussed in Sec. 778.417 are met and:
(1) The piece rate is a bona fide rate;
(2) The overtime hours for which the overtime rate is paid qualify as overtime hours under section 7(e) (5), (6), or (7);
(3) The number of overtime hours for which such overtime piece rate is paid equals or exceeds the number of hours worked in excess of the applicable maximum hours standard for the particular workweek; and
(4) The compensation paid for the overtime hours is at least equal to pay at one and one-half times the applicable minimum rate for the total number of hours worked in excess of the applicable maximum hours standard.
(b) The piece rate will be regarded as bona fide if it is the rate actually paid for work performed during the nonovertime hours and if it is sufficient to yield at least the minimum wage per hour.
(c) If a pieceworker works at two or more kinds of work for which different straight time piece rates have been established, and if by agreement he is paid at a rate not less than one and one-half whichever straight time piece rate is applicable to the work performed during the overtime hours, such piece rate or rates must meet all the tests set forth in this section and the general tests set forth in Sec. 778.417 in order to satisfy the overtime requirements of the Act under section 7(g) (2)."
However this does mean keeping track of pieces during overtime hours SEPARATE from those during regular hours AND I am not sure what happens if an employee is in the middle of a "piece" on the split between overtime and regular. I also imagine there are some tests that you would have to do to prove that the employee is making at least minimum for regular hours and 1 1/2 minimum for overtime hours. So you would still need to track total hours worked and compare the piece rate wages to hourly wages on a consistent basis. I am assuming you have already done some analysis on this part to pick the piecerate payment method in the first place
I know a really really good FLSA Wage/Hour consultant who retired after quite a few years with the DOL Wage and Hour division. He just helped me out with a sticky wage/compensation situation. If you want his information, email me at resumeHR2007@gmail.com.