I agonized over what to do - If anything. Definitely read all the literature out there about potential action items.
Was thinking about visiting again (this week, but didn't), having a few short emails of support from alumni (not ultra-influential alumni tho; just successful people)...ultimately decided against that too because generic/they don't know me that well. Also decided against an "update letter" written by me b/c IMO only worth something if a material, material thing has changed in your profile.
My only points of contact have been: 1) directly replying on March 25th saying I wanted to be on waitlist and HBS was unequivocally #1 for me (so they at least know if they called, I would say yes) and 2) indirectly my CEO (not HBS alumni tho) sent a 1.5 page "letter of support". That could end up coming back to bite me in the ass if was annoying/solicited, but for my own personal "I did all I could" I had her send it.
Really I don't know the playbook and what works tho.