The conference play command doesn't use the same core functions as the regular playback app so you can't use !-separated lists. However, you could use loopback to call into the conference and use the playback app. Just make sure that you disable the enter and exit sounds. Off the top of my head I'd say it looks something like this:
originate {conference_enter_sound=silence_stream://1,conference_exit_sound=silence_stream://1}loopback/3000 &playback(file1.wav!file2.wav!file3.wav)
before telling that it doesn't work. :)