Well, whatever you come up with, let us know! That sounds like a cool invention, however, i can see numerous issues right away that will likely make it unfeasile to build.

Most things would need to be weighed down otherwise they float, depending on the displacement of the product, it may need a lot of weight to keep it down, unless it's steel, in which case you probably won't get it out again due to weight.
Your "feet" under the platform would have to disperse the weight of the platform, the weights and people on the platform evenly and without damaging your liner.
The platform should not move for aforesaid reasons as well as chaffing the sides of the liner too.
Floor would likely need to be perforated otherwise you'll have a hard time getting it into and out of the pool.