I just found out that Finnish has 29 letters—the English 26 plus Å, Ä and Ö. The Ö is used as the twenty-seventh rehearsal mark in Sibelius’s seventh symphony. Dorico, not unexpectedly, does not have this as an option for rehearsal marks in excess of 26. I’ve faked it in my Dorico score with a text object, which looks fine (though I haven’t dealt with parts, and don’t intend to, except as a fact-finding exercise to see how it would work)
But wait, there’s more…
In the Sibelius score, Rehearsal Marks are placed above the top staff and below the bottom staff, and all tempo indications are placed over the top staff, and over the string section. In Dorico I don’t know how to achieve this—I can have either Rehearsal Marks in all 3 spots and tempo over the strings, or Rehearsal Marks at top and bottom with no tempo indications over the strings.
Have I overlooked an option for this?
If not, could this be added to your list for future implementation?
I’m curious how a non-Finnish-speaking conductor would tell a non-Finnish-speaking orchestra to go to that rehearsal mark … 
One more thing we can pin on “Sibelius”? 
Why, “Ö”, naturally!
According to Wikipedia:
The precise pronunciation tends to be between [ø] and [œ].