Could both harnessed magic and tethered magic be designed into a spell that allowed for the spell to end when the target comes into contact with something specified in the spell? I am designing a magus that believes redcaps read his correspondences and wishes to develop a spell that transforms his letters into an object which only turns back into the letter when the object comes into the hands of the proper recipient. The muto spell would be tethered to end the harnessed spell when the recipient touches it (an arcane connection to the recipient is probably needed).
I think it's possible, but yes, an arcane connection is probably needed. You'll need Moon duration at the least. Whether incorporating arcane connection to your recipient in the letters you send is something I'd recommend, on the other hand... You could consider effects similar to those in the Tytalus chapter, which are more straightforward.
Thanks for the Tytalus suggestion, I assume you mean Reveal the Hidden Words, charged item. That works, but has the disadvantage of having to create a charged item for each correspondence. That’s a lot of time if you are writing several letters if a full season of work can get you enough ink for only 5-6 letters. I guess you could always hand wave things and assume that one charge will equal 10 pages of writing, so if you keep your letters short you will have more ink for the invested time. A similar kind of spell would be more efficient in the long run.
Why not just change the letter into something with Moon Duration? The recipient just needs to wait until the duration expires, at worst.
If you feel this ain't long enough, first, read this:
=> you could design a Ring/Circle spell of the Type I persuasion (affects what is inside the ring at the time of casting, even if it leaves the ring, so long as it is intact) that changes your letter into, say, a lump of wood.
Then, you need, say, a small wooden circle.
Put the letter into the circle, cast the spell.
You just send the lump of wood, and then, either, after a while, you break the circle, or your correspondant just casts some small Perdo Vim effect to cancel it, turning the lump of wood back into a letter