So my pcs are all taking their oath at Hibernian tribunal meeting of 1221. They want to get to know the Redcaps better in order to get some ideas as to where best to establish a covenant. The redcaps want to have some fun with their new sodales and plan a good hearted (perhaps slightly embarrasing) prank on the new magi.
Any suggestions?
Away from books and nearly without time. But tmk Hibernian apprentices before their gauntlet are loose cannons and not responsible for their actions.
Do some Redcaps have a bone to pick with a Hibernian magus, and think about sending some apprentices after him? Then they just need to hide their own identities well from the apprentices and still look convincing and needy.
My players have already had their macgnĂmartha (Hibernian adventure period before taking the Oath). They will begin as new members of the Order. I'm looking for ideas akin to the Subrosa #12 adventure for grogs during a tribunal. Light but potentially embarrassing, where in the end all the redcaps are giggling and they realise they've been had.
So have their sworn their oath already?
Yes.
Tell them where there is vis to be harvested, and that the Tribunal will reward them with a share if they do so. This particular vis source is one guarded by spiteful fae who specialise in petty vengeance and curses. Normally gathering vis from this source is regarded as an onerous chore, best suited to those with a strong Parma Magica or a lot of Faerie Lore. Still, if the redcaps can find a group of willing suckers to do it instead, it's a learning experience right?
Other alternatives - send them on a wild goose chase, so they need to find the invisible thread. The invisible thread needs to be found with the magical lens, so they need to find the mage who has that. He will only lend it to them if they find a five-leaved clover. Keep adding ridiculous tasks to the list until they are able to go looking for the invisible thread, and then have someone hand them an empty spinning wheel which has an illusion of thread that only exists to the sense of touch cast on it.
Just dare them to do crazy stuff - I believe the Sub Rosa adventure you are referring to involves a forfeit of kicking Archmage Darius up the ass, yes?
Love the invisible thread idea! Yes, the Sub Rosa adventure does end with a grog having to kick Darius in the rear. 
When camping as a kid, we would send new campers on a "snipe hunt" or to look for "left handed smoke shifter"
Of course too many might recognize this, so perhaps finding the "Widdershins Air Displacement Rock" or a hunt for the "Bladenberg" (A hald bear, half panther but only 1 foot tall, but worth 3 pawns of Corpus vis)