I don't think you can find it all in one location, and you need to post your comment from the First Draft, but you also need to have information from the First Draft Report to put in your comment.
As far as I can tell you can only comment on the actual First Draft and if that is the case there is no way to comment on a rejected PI.
What a huge step backwards for the whole code revision process.
I just submitted a PC.
AFAICT, the process is only different than a PI submission in that 1) comment will be from NFPA 70 view which is identical to the First Draft Report except indexing on left is more refined, and 2) there is an added step which requires adding at least one Related Comment or Related Item—for which Public Input, First Revision, Committee Input, Correlating Revision, Correlating Note are the choices along with their corresponding number.
What this amounts to is, if you are commenting on a rejected PI, you will have to revise the NFPA 70 text (whether it be a revised section or unchanged section) to mimic the PI (with perhaps any additional changes to the PI) prior to the Add Related Item stage. The difficult part is accessing the related item information while in the revision stage. As such, I suggest anyone submitting a comment on a rejected PI to gather all pertinent information in another application (e.g. have the Public Inputs with Responses PDF open to copy from) during the process.
I also noticed "Add New Section" is available from the NFPA 70 view. I did not use that option at the PI stage or now, so have no idea what is involved in that process.