However, I’m wondering if another customer base can be someone who has bank bill pay but wants to float the funds until the check is cashed. Maybe they don’t have the actual funds yet but want to write a check against funds they expect to have soon (risky but people do it).

Do you restrict writing checks that are for an amount greater than the current account balance?

There's no reason to. You can already write a check for more than your account balance. The service also generates the checks using your account number, so they're not taking on any credit risk in case your account is overdrawn.