I think it is in the rules.
The Wizard's Rules 10 · PLAYING AND DRAWING CARDS wrote: Except for resource long-events, you may play resource cards anytime during your own turn unless specifically prohibited by
the rules or the cards themselves.
This is a foundational rule applying to every aspect of the game.
My interpretation is that the Rules (eg, one character only per turn may be played during the org phase) prohibit play of additional characters beyond 1, or the play of characters outside of the organization phase. This rule is even in bold font. Here, I interpret anything not specifically allowed by the rules to be "prohibited" by the rules, basically by definition.
Despite the lack of proper wording, A Chance Meeting was intended and ruled to override the Rules. This is unfortunate. It really should have explicit errata.
Without more explicit wording on the card itself, the quoted rule clearly prevents a card from overriding this rule.
There is no indication that additional play allowances were intended by Open to the Summons or Thrall.
To Konrad's other point in the linked thread, I don't agree that all short events create their own playable allowances to override any rules with the need to say so explicitly (as that goes against the rule above).
Other short events suffer some similar issues. Like an attempt to play Old-Road outside of the site phase based on similar reasoning as A Chance Meeting. However, this is not allowed by the fundamental rules of the game (unless you are a chance meeting, or we have come to kill).
CoE 11 wrote:3. I can't find anything that tells me I can't use Old Road in the org phase, or even that it results in the
Haven or the remote site tapping.
*** CRF Site Phase/General: "Items, factions and allies must be played during the site phase."
*** CRF Site Phase/General: "The site taps upon successful play of the resource that would tap it."
That factions are (only) playable in the site phase is a rule of the game. Old Road cannot override this, even by virtue of being a short event.