Programmable progress bar

Is it possible to programmatically change the size of a rectangle shape in Storyline? I want to build a custom progress bar which tracks from the beginning to the end of a layer's timeline so that the user knows how long the content is on that layer and knows where they are.

We can't use Storyline's built-in seekbar: it doesn't fit our requirements because the built-in seekbar appears automatically on every layer in a slide and we only want it to appear on one of 2 layers we have on our slide (one has a timeline, the other has just static text - we don't use the timeline, so a seekbar isn't appropriate on that). We can't use states because we might have content which is say, 2 minutes, long and we couldn't have that amount of states, that would be a nonsense. All help very much appreciated.