Isn't it much simpler to create the HU in the system (without the TO) and then assigning it to the TO after TO creation? TO number should be irrelevant in the pallet label because you can extract that information from SAP if you want to audit the process.
I think you should reconsider your process instead of updating SAP tables as there is no BADI/BAPI that does what you want (and the process doesn't make much sense from a best-pratice point of view, you shouldn't have a TO in the system that isn't actionable).