I would think Tesla would be unlikely to allow user choice to down-rate the charge rate at a 90kW super charger for a few reasons:
1. They know better how fast your battery can take a charge based on how it's been used recently and over it's life. Why offer you the chance to second guess them?
2. That supercharge station represents a big capital investment. Why would you want it to reduce the number of cars it can potentially charge per day because you let a user turn a 30 minute charge into a 2 hour one?
3. It's not good for the Tesla brand for cars at supercharge stations to be on charge longer than they have to be ("Oh they say it's 30 minutes but I saw one charging for over 2 hours the other day")
4. Other Tesla customers will be upset if they turn up to fast charge, there is a car already there, and it ain't gone in around 30 minutes or less.