Use an old android phone. The android version of SIREN will control the radios volume.I'm using a "de-commissioned" iPhone wtih the Siren app as a remote head for my 536. It "sort of" works okay, but some improvements are needed in the Siren app. The biggest beef is that I can't control the scanner's volume from the iPhone.
I'm sure Uniden was thinking a combination such as this would replace the remote head that was available for older scanners. It just hasn't worked out as good as hoped for.