It is quite common that scores that use alphabetic characters as rehearsal marks omit the letter I, presumably on the basis that it is easily confused with J. This was possible in sibelius by creating I at a dummy location and then hiding it. Is there a similar facility in Dorico?
In Dorico you can override the index of the rehearsal marks. That means, if you select the rehearsal mark , and set its index in the bottom property panel to 10 (j is the tenth letter of the english alphabet) it will change the rehearsal mark to [J} and all subsequent rehearsal marks will be updated accordingly.
That’s workround is OK until you add or delete a rehearsal mark before letter J. 
I think there should really be an option in the Engraving rules to include or omit letter I in the alphabet. Lilypond has had the options of “letters” (the default, without I) and “alphabet” (the full alphabet) for a long time.
Thanks for this suggestion, which I will add to our backlog.