epoxy 'n' mat
The binder referred to is sprayed on to the mat in manufacture to hold the loose strands together. It is a styrene and is disolved by the polyester just like a styrofoam cup, allowing the fibers to splay and move independently. In the same way that you can lay-up over styrofoam using epoxy the mat will not achieve it's wet dishcloth state if the binder is not desolved.
If you are concerned about the added strength, flexability and adhesive qualities of epoxy you might think of using a vinylester resin which falls somwhere in between epoxy and polyester in most catagories and I believe has no issue with the soluability of the styrene binder.