I don’t think that it’s possible at the moment with slider animations from Grasshopper.
If you export the frames with a neutral, solid, unique coloured background, you can use an image editing application with batch editing capabilities to remove the background.
What you’d do is set up a workflow that can be applied to an entire folder of images.
For each image you’d select the entire background by colour - it needs to be unique so that no relevant other area gets selected -, then you’d have to simply delete the background, or invert the selection and apply a mask. At the end, simply save the file in an appropriate format.
Once the workflow is recorded for one, you can apply it to all images in a directory, and thus process hundreds or thousands of them.
Results may vary, but if you dial in your selection options, you can get a relatively good output.
I think that for instance Vray allows you to render with a transparent background from Grasshopper, but I don’t personally use it.