I asked the Diamond Line to email me copies of my upcoming reservations; when they did, the "comments" section was revealed. So in the past if you added comments and want to see if they're still there, or are curious about any comments added, have them email you a copy. (Which appears to be different from the typical email versions you can request yourself online.)
Curiously, one of my upcoming reservations says "HH\\DIAMOND UPGRADE,SUEIT IF AVAL."
Hopefully they plan to give me a suite and not sue me