After much reading, I discovered a possible solution to the 805 trigger. I'm not at home so can't test it yet. I'll give it a try after taking my son trick or treating.
Zone 2 can be used as a preamp only. This means one does not have to forego the use of the surround back channels (as I had originally thought).
If you press the Zone 2 button on your remote, any subsequent button presses will be for zone 2 (until your choose "Receiver" again). Thus, press zone two, then press the power button, and this will activate the zone two trigger. I haven't sorted out whether you need to first enable something in the setup or not - it may be that you only need to turn on zone two in the setup if you plan to use the SBR/L amps channels.
I'm using a Harmony 890. I'm hoping I can "learn" some new remote codes to make this seamless. By default, the zone two commands are not available. I did find the remote hex codes, but can't seem to program them in. It can only learn by line of sight.
It is possible, however, to add the Onkyo Zone 2 as a separate device. This is a less elegant, second option to learning the zone 2 codes as custom commands for the primary Onkyo device (in the mind of the Harmony remote).
If I can get it to learn the codes, then it should be an option to add the extra commands to the on/off sequence for the device.