Leaving the committment issue aside for a moment . . . one thing that worked for us was to include, from the beginning, 1 or 2 non-paying players to the roster, to use in just such an occurance. They were kids who were good players, but the parents didn't have the money to play select team fees. So when we needed them, they came along and played and we could field a team. It wasn't quite the team we wanted to field, but we fielded a team.
Although it sort of 'solved' the immediate problem, the fact that some regular players chose to bail on the team in an important tournament with national implications still rankled the other players on the team.
Bottom line for the tournament though, is go play it if you want to and bring some kids who want to play. Generally if it is an independent tournament you are not bound to the roster that you have for States/Regionals/Nationals etc. Usually a tournament roster is submitted when you register, for that tournament only, so you could add them when you sign up for it. But if the 'extra' roster kids are on your roster from the beginning, they'll be eligible for anything, including States/Regionals/Nationals.