I don't see any legal issues with that. Texas is not one of those states that considers PTO to be wages. At worst, you might have to reinstate the time if they get the timesheet in within x amount of time. In fact, you might make that part of the policy. "Effective May 1, 2015, if your timesheet is incomplete or missing the applicable pay will be deducted from your PTO balance. If the corrected timesheet is submitted within 3 work days of the issuance of the applicable paycheck, the PTO time will be restored. If the corrected timesheet is not submitted, or if it is submitted more than three work days after the issuance of the paycheck, the PTO deduction will be permanent". That's just off the top of my head; you'll want to work on the language. But if I am not mistaken, something on that line should past muster everywhere but CA and if you're careful about the wording, in CA too. DAWW should be able to say if I am mistaken or not.
ETA: You'll also want to have a policy in place for what to do if they do not have sufficient PTO to cover the balance.

