I'm assuming that the players do need some kind of reminder, and that just stating "This map was found" might momentarily confuse them. I suggest using "our" to sum up "we worked out."
In our fictional history, this map was found in the library of Fred the
According to our fictional history, this map was found in the library of Fred the
If you think something like "ostensibly" could replace the entire phrase according to the fictional history we've worked out, then perhaps you think the players will know you're talking about a fictional history and don't need a reminder at all.
The problem with a single word like "ostensibly" before "found" is that it will likely be interpreted as modifying "found" within the history:
Ostensibly found = the provenance of the map is uncertain—it was said to have been found in the monk's library, but this may not be the case.
Scare quotes (This map was "found" in the library of Fred the Monk) might convey your meaning, but IMO seem a bit silly and a roundabout option. They could also be misinterpreted in the same was as ostensibly found.
The only single words I've come up with is:
Storywise, this map was found in the library of Fred the Monk.
In terms of a story or narrative. wiktionary
But I can't say I like it. It's also possible that an adjective you like won't have an adverbial form that can function as a sentence adverb.
I don't think contextually works in this position either. You might make a case for it, but I don't think the players will understand it.