See if you can figure this out. Do we call Abraham Avinu z”l? What about Moshe Rabbeinu? What about Rambam? What about The BESHT? What about Yaacov Ben Rivka? What about the first man Adam? What about Mamela Rochel? What about Sarah Imanu? Not everyone in Jewish history has that designation.
Many Holy Tzadikim can be prayed to any time and anywhere. Artscroll Siddurs contain prayers to pray at kevers/gravesites or in the privacy of your own home or Synagogue with others.
I don’t write shlita on correspondence to The Rebbe’s OHEL I just state my full Jewish name listing my Mother’s name
And begin writing Dear Rebbe, and so on and so forth. When reciting prayers to Mamela Rochel I never make a title I just pray for her intervention in my desired prayer need. It’s completely up to you how you will feel comfortable to address The Rebbe or any other Tzadikim.
We have a very rich Jewish prayer life. Make use of it and see for yourself the positive results from it.