Jason,
UIL Exception 28, referencing rule 3-1-1-e, makes it only RECOMMENDED, that the toss be conducted at 3 minutes before kickoff, with captains. By this exception, coaches that know this exception, have the ability to have the toss at any time before kickoff, and between coaches, captains, grandmothers, parakeets… whatever they want.
Best not to ask. Just let them know that you’ll call for captains at 5 minutes, and conduct the toss at 3 minutes. Rarely get an objection.