I see no reason why the orientation of an
Arcane Mark could not be altered on casting. Imagine casting this on a piece of paper -- will the mark always be "right side up", no matter how you turn the paper?
Anyway, it's neat that a bunch of people thought of using the
Arcane Mark as a cipher, which is exactly what a high-ish level NPC did in one of my old campaigns.
His mark was a staff with a distinguished tip and end. You could easily do the same thing with anything that has a distinguishable front and back (like an arrow |--->).
The cipher was created by placing the mark in any of the 12 clock positions and either putting the mark "high" or "low"; that is, the four horizontal positions
would all have different character representations. This allows for 24 distinct characters, which is more than sufficient. To make the cipher a little less trivial for the players (since this is a direct substitution cipher, you'd be able to figure it out with a decent sample size), I defined some orientations/heights as common digraphs (two letter pairs like "th" or "in") and omitted a few rather unnecessary characters completely ("c" and "q" can be spoofed by other letters,
"z" and "j" are very infrequent and could be similarly replaced). This was also nice in that it lowered the number of spells to cast to properly mark a word or phrase (oftentimes the word or phrase was truncated as well, or written in text-message style).
At first I thought the angles might be a little confusing (eyeballing 7 o'clock vs. 8 o'clock seems tricky sometimes, especially with small lines), but the players had no problems on that front, probably since I helped them out by printing the dang things out when they found them.
The messages were never really important, but sometimes they were nice bonus clues. One of the players (the one who had permanent
Arcane Sight, heh) took notes of any diagrams they found and caught on to the cipher relatively early, and it became a source of fun information for them. I guess people enjoy cryptoquotes in the paper, and this is a perfect example of that.