I have never seen signs at any of the Resort playgrounds restricting their use only to Guests of that particular Resort as is the case with Pool signs.
I am assuming the different Resorts' insurance policies would cover ALL Guests and not just Guests of that particular Resort should there be an injury while playing and it was determined there was faulty "equipment",etc.
By the way,AoA has an AMAZING playground in the Nemo Courtyard and The Lion King playground at AoA is supposed to be "off the charts" when it opens.