You can use an autonomous selector (SendableChooser) even if you are not using the Command system.
You can place any type of data in the SendableChooser, not just Commands, because SendableChooser.addObject() takes any type of Object as its second parameter. You could use numbers or strings instead and then use these to decide which autonomous to run.
The SmartDashboard does lack a slider control by default, but it fairly easy to write an extension to add this capability.
Here is the one I wrote (both files are necessary):
Slider.java
BindableSlider.java
I also attached a compiled JAR of this extension (inside a zip because CD doesn't allow me to upload a JAR).