In reply to Steve Long:
I think the project is a wonderful idea and wish you the best in carrying it out!
I do think the concept of "best practice" is a fraught term that may not be the most productive approach to disseminating the "wisdom" you refer to. The simple adjustment of making "practice" plural would be at least a step in the direction of acknowledging that most practices are context-dependent .
The example of rappel back-ups has already been raised, so consider the options.
1. Backup or no backup, including how to think about devices that (may or may not) lock when released.
2. If backup with friction knot, then (a) using Prusik, Kleimheist, autoblock, Bachmann, Hedden, etc., and (b) friction knot above or below the device.
3. If below the device, then with friction knot on leg loop or on belay loop.
4. If friction knot on leg loop, then extending or not extending the device.
I think each of these options is worth a bit of discussion in which the pros and cons are elucidated, and in particular potentially surprise "gotcha's" described (e.g. putting the friction knot on an adjustable leg loop in a position that might, when loaded, release the buckle).
With the pros, cons (including "gotcha's") described, I wouldn't find it too offensive to proclaim that if a backup is to be used, then the best practice would be to extend the rap device and install an autoblock knot on the belay loop, because the analysis suggests that this method has the most advantages and fewest drawbacks. The point is that if a recommendation is given---and there is no reason why various alternatives might not be presented as in general equally effective---then it should be in the context of a consideration and analysis of all the alternatives, and not as a prescription from on high to be incorporated into an uncritical climbing catechism.
Among the many good reasons for such an approach is the suppression of the "reinvented square wheel" phenomenon, in which someone without the requisite experience revives an already-discredited (but not mentioned) practice in the hopes that their "discovery" is some kind of improvement.