Guide services are are currently prohibited from guiding technical and/or back-country routes within Zion National Park, this includes the Subway. Zion does allow front-country guiding but the reality is that most of these trails/areas are very easy to see on your own. The Subway should be treated with respect as it does involve some technical sections, swimming, down-climbing, route finding, and strenuous hiking.
If you are not familiar with technical canyoneering the surrounding areas offer high quality canyoneering routes for visitors to experience with a Guide Service. It is also possible to take an instructional canyoneering course so you can do routes within the boundaries of the park on your own. All zion canyoneering routes require a back-country permit.