If you are just stabilising the position of an object, it could be accomplished like this:
- Track the footage using the tracker. Window -> Tracker
- Set it to 'Track Motion' not 'Stabilize Motion'.
- Make sure 'Position' is checked.
- Run the tracker.
- Once it is complete, hit 'U' with the layer selected to expose the tracked keyframes.
- Create a new Null layer. Layer -> New -> Null Object
- Add a 'Slider Control' effect to the Null layer. Rename it to 'Amount'.
- Press 'P' to expose the position property.
- Alt + click the stopwatch to create an expression.
- Paste in the expression below:
// The original position
var origX = 646; // Change this
var origY = 330; // Change this
// Your tracked footage
var track = thisComp.layer("Tracked Layer").motionTracker("Tracker 1")("Track Point 1").featureCenter;
// The difference between this frame and the first frame
var dx = origX - track;
var dy = origY - track;
// Add the difference back to the original
// Multiply this by the amount you want to achieve
var newX = origX + dx * effect("Amount")("Slider").value;
var newY = origY + dy * effect("Amount")("Slider").value;
[ newX, newY ]
The origX and origY values should be set to be the same x and y value of that of the very first keyframe of the 'featureCenter' property on your tracked layer.
You will also need to rename the layer referenced in the 'track' variable from 'Tracked Layer' to the name of you tracked footage layer.
You can now keyframe the Amount slider values, keeping it between 0 and 1. Where 1 would be fully stabilised and 0 would be no stabilisation.
I hope this is close to what you were hoping to achieve!