I cannot find CH8_OPT or any of the other CH OPT parameters using QGC.
I had expected QGC to allow editing of all ArduPilot parameters . . .
I think they’re called “RCx_OPTION” now…
http://ardupilot.org/copter/docs/parameters.html#rc10-option-rc-input-option
Confused here 
This section of the wiki mentions CH options
http://ardupilot.org/copter/docs/channel-7-and-8-options.html
Is this section now obsolete? RCn_OPTION has about 10 more choices… . .
BUT QGC does not find these parameters either!
This depends on what version of firmware you are using it isn’t really a QGC issue.
Arduplane 3.9.11 on Chibios.
You are referencing Copter parameters and you have Plane. If you want Rcx Options for Plane you must load beta. Currently V4.0.0beta4. Then you will have these options available.
TX for clearing that up.
Next dumb question – can I install the latest v4 beta via Mission Planner?
Yes. You have to go to the about page of MP and select ‘check for beta firmwares’ or something like that. and restart MP. Then the Betas should show up on the load firmware page
edit - i am not too familiar with QGC, but if it can load custom firmware from a file, you could also download the beta firmware files
Yes, or QGC. With MP hit the “Beta Firmwares” button and it will populate the page with the latest beta.
I wish the wiki would differentiate parameter sets between the various firmware releases…
QGC provides the option to flash Stable, Beta or Dev builds as well as pointing it directly to a firmware file. Just pick which version you want from the dropdown.