Does this ripoff of Knox's Decalogue suffice?
I. The entity must be mentioned earlier either IC or OOC if it is to be suddenly involved, its involvement must be justifiable with that information; Suddenly being rescued or beset upon by flying pigs from the frozen plains of hell demands justification.
If you have things that may pop in, at least make sure everyone knows that they may be involved rather than showing in previously unknown entities.
II. Any sufficiently powerful entity must have a reason not to intervene in a decisive in plots; Otherwise there would be no need for characters to do anything.
Such as being busy, apathetic, having an odd sense of morals, etc.
III. No specific philosophical outlook may be favored over another; There are enough arguments already without getting epistemology involved.
Observer interference in void is all fine and dandy as long as empiricism, rationalism and the like make still be applied.
IV. No hitherto undiscovered supernatural phenomenon may be used; After all, doing so would seem like an ass pull.
You could say it's theoretically possible, allude to it's existence or provide enough information for people to recognize it as a possibility, don't just shove it in there.
V. No Crossover must figure into the story; There is a crossover ban after all.
People can go coplay and mimic whoever they want, just don't shove other fiction into this.
VI. No accident must ever help the players or antagonists, nor must any party ever have an unaccountable intuition which proves to be right, unless that is their ability of course; Again, that would be an ass pull.
Needless to say Will and to a lesser extent, Azerfernia are covered by the end bit, this also helps prevent exploitation of meta-knowledge.
VII. Any lore or plot information is to be presented on request when its examination no longer risks spoiling or causing metagaming; Transparency and whatnot.
This also helps people figure out how parts of lore or plot they liked were constructed and replicate that.
VIII. The characters must not use abilities or skills which had previously produced for the inspection of the players; That would be Mary Sue-ish, would it not?
Not as in displaying more of their current ability at higher output or an application of it, as in, suddenly using an entirely new ability without warning, emotional trigger is not an excuse.
IX. All relevant information that may affect other realms are to be disclosed to the players running those realms; Seriously, that needs to happen.
It's best to know the full consequences of your developments after all.
X. Semantic abilities or similar phenomenon generally, must not appear unless the lore duly prepares for them; If creative interpretation may be used, there should be a hint to that.
For example, Wall Street has an entire treatise entitled 'On the Economics of Non-Sentient Entities' to justify its rather odd applications of economics.