Ah ha! That's in the Reaper pack, and I don't have it here (a cohort owns it). According to its description, does it affect any card placed on the board or only Adventure cards?
Overall, considering Spells, Events, etc., there is no way to avoid an Adventure card being interacted with in some way, so giving them new classifications isn't going to keep them in place. The only way to do the permanently is if they are not an Adventure card or somehow completely outside of game mechanics. That precludes them being "drawn" and requiring them to be "placed" before a game begins. And as much as your new mechanic counterbalances, its beginning to be too much text on the card not related to what it does.
It's all beginning to sound like adding too much that's not standard to the game in order to keep it in place.... hence, back to square one, so to speak. I'm probably going to go with Strangers and Places like I originally did in ITB. And let the cards fall where they may.
Later, I may create alternative cards that are the same and place before a game begins. But for now, it just seems like too much trouble.