My suggestions for a McGuffin have been superseded by the last post, as they're not very volkisch, but I'll post them anyway.
The Lance of St George:
St George was a Roman soldier born in Cappadocia, living around 300 AD. His lance may actually appear as a more of a spear, and could be found alongside the Spear of Longinus in an occult weapons collection, perhaps even being mistaken for it due to its vintage and origin. To otherwise cloud matters, it could be referred to as "The Cappadocian's Spear/Blade/Relic". It could be a justification for dragon-like or demonic entities appearing in the vicinity- give it summoning and control powers. Note that St George's day is April 23, so that might be useful too. And this would be a particularly significant relic for the British to recover. Perhaps it could be a sword, or it could be piercing through a stone (a petrified dragon's heart, liver or gall), if you want to raise some confusion between it and Excalibur. The Relic could either be in its original form, or it's been weaponised as the core element of a massive deiselpunk death machine.
The Last Descendant of Christ: Taking a leaf out of Leigh, Lincoln & Baigent's
Holy Blood and the Holy Grail (which of course was the basis for
The Da Vinci Code, but you can ignore that), and also riffed upon by Garth Ennis's
Preacher (which is about to become a TV series, so I'll try not to drop any spoilers)- the bloodline of Jesus has been maintained throughout the centuries by a secret cult. Their last hiding place had been in the Spanish town of Guernica, so after German agents had captured the Descendant the town was bombed into oblivion. They have had the Descendant in their custody since then. This means the McGuffin is actually a person, making escape that much more difficult. The Descendant could be any age you choose, from an unborn child in his mother's womb to a decrepit old man, to further make things interesting. You could also check out how Ennis portrayed the Descendant for another twist, but that way lies spoilers. To make things hairy in the vicinity, the Descendant is protected by a legion of angels, ghostly saints or apostles that your team has to contend with. Your preacher's son PC might have been chosen for the mission with this target in mind.
The Minder: The McGuffin, whatever it ends up being, must actually be taken together with a scientist, academic, doctor, engineer or occultist who is needed to protect it or keep it under control.
ETA:
Napoleon's Head: '
Nuff said.