Yeah, with PS2 locos, it would seem to remotely control the TIU would be the easiest route.
But, upon reading Mark's site, it seems that while you have interfaced a computer to the TIU, there is no phone app to interface to the software, maybe I missed this. (let me know please).
In JMRI, it has a "server" and apps for iPhone and Android. So if you have a computer to use, then the interface, server and apps are all free. Of course it only interfaces to DCC though.
Greg