So taking a peak around the crusader event files they have events for finding the spear of destiny/holy lance and the holy grail. It seems that we could basically copy/paste these with minor alterations to simulate the finding of the true cross or the crown of thorns. The current events give 100 and 200 prestige respectively for finding the spear and grail and finding these two relics should be worth more prestige (though obviously not a game breaking amount of prestige) It should probably cap out at 300 prestige. If an event can bestow a title to a character finding the true cross it should for instance say "Geoffrey guardian of the true cross" or something to that effect. Perhaps the other events should also be modded to that effect.
Next it would be nice if there were a random event whereby the holder of the true cross (we might assume the True Cross continues to reside with the Kings of Jerusalem as they did historically) may loss it if they lose a large battle with a Muslim army so as to simulate a Battle of Hattin kind of event though it should be a low percentage randomization that can only be triggered if the true cross is held by a ruler. It is well attested that medieval christian rulers would bring relics with them into battle so as to gain divine favor.
Keeping on the issue of relics it is well attested to in Jonathan Riley-Smith's book that the sale of relics was, if not common, an accepted practice for lords with financial issues to use. Quoting from his book (pp. 190-191):
"In 1238 Baldwin (II), badly in need of money and military support, raised with Louis (IX) the transfer to France from Constantinople of one of the best-known relics of Christ's Passion, the Crown of Thorns. All his life Louis was devoted to relics: he avidly collected them; he built churches to house them and made gifts of them to favoured institutions; his travels were punctuated by visits to shrines. His response to the offer from Baldwin was immediate. He sent a mission to Constantinople, which discovered on its arrival that the Crown of Thorns had been pledged to the Venetians. The mission negotiated the transfer of the pledge to Louis for 135,000 pounds (at ta time when the monarchy's annual budget was 250,000) On 11 August 1239 the king and his brothers met the reliquary at Villeneuve-l'Archevêque and carried if barefoot to Sens. Taken by boat to Vincennes, it was again borne by them into Paris where, after being exposed for the veneration of the citizens, it was transferred to the chapel of St. Nicholas in the royal palace.
Louis had already learned, probably from Baldwin, of the dispersal of other relics from Constantinople to Palestine, including a fragmetn of the True Cross which had been pledged to the Templars for a huge sum, and a phial of the Precious Blood. These were redeemed on his behalf and were brought to Parris in 1241. In less than three years the king had acquired a major part of the famous collection from the imperial treasury in Constantinople. It included relics relating to almost every stage of the final hours of Christ's life on earth and many others besides. It rivaled even the ancient treasures in Rome and its provenance was in thirteenth-century terms unassailable. One of several pieces of the True Cross, for example, was probably the famous fragment which had always been stored in the Great Palace of the Byzantine emperors and had helped make Constantinople the chief centre for distribution since the sixth century. Baldwin made the collection over to Louis absolutely in June 1247. To house it Louis had already begun to build the Sainte-Chapelle, which was completed within a decade."
In addition I'd like to point out how Louis treated the relics...
"It is probable that Louis had heard of the taking of the city (I think Riley-Smith refers to Constantinople and the fall of the Latin Empire in 1261 here) just before he fell into an illness so severe that it was said that at its height Blanche of Castile ordered the relics of the Passion to be brought for him to touch. It is remarkable how often these relics played a part in his crusade preparations thereafter. He timed the solemn tournée of his domain before departure on his first crusade so as to be present at the dedication of the Sainte-Chapelle on 25/6 April 1238. As late as June 1270, while in Aigues-Mortes preparing for his last crusade, he was making dispositions relating to the care of the relics in the Sainte-Chapelle. When he was planning that second crusade, John of Joinville, who was going to register a protest against it, found him in the Sainte-Chapelle. 'He had gone up to the platform here the relics were kept and was having the fragment of the True Cross taken down.' The relics answered to the needs of a man with an exaggerated, even histrionic, devotion to the cross which was so well known that Humbert of Romans referred in his advice to preachers to the 'king of France carrying the holy relics of the Crown [of Thorns] and the Cross of the Lord on his shoulders in his chapel'."
There is amble evidence right in these quotations to set up a good half dozen or more events relating to relics alone and I'll try and talk about what I think would work tomorrow. In addition if you have any questions relating to research/etc I'll be happy to help as I've still got access to JSTOR through my university.