I suggest that the OP does not understand the charter business. The airline's customer is the charterer, which is normally a tour operator rather than a travel agent. That tour operator buys the whole capacity of the aircraft for the duration of the contract, and assumes the financial risk of selling (or not) the seats either direct or via travel agents. The flight schedules are, of course, known to those selling agencies.
Since individual seats are not sold direct to the public, by the airline, there is no need for a published flight schedule.