It is not normally possible to access the toilet during the flight whether long or short haul. The majority of toilets on a plane, whether they have a disabled symbol on the door or not, are small and only have room for one person (the user). The airline doesn’t usually carry an aisle chair for you to use to get to the toilet. Consider attaching a night bag if you are on a long flight which can be concealed inside a carrier bag and placed under your seat.
Please note, the cabin staff are unlikely to be prepared to dispose of urine for you. Your individual requirements for your personal care should be carefully thought through before travelling.