I am glad that I saw your post before you deleted it - because I think the idea with an extra icon for comments is nice.
Using the 3rd field for comments is tricky and no long-term solution. Reason: Some commands syntax check that the field is empty when no 3rd parameter is supported by this command. This is to help to avoid “stupid” errors, like writing CHECK | xpath=... | FALSE when the real Selenium IDE command is UNCHECK. This example happened to me just yesterday.
The classic Selenium IDE has a comment parameter for each command (see below). That would be easy to add. But what we struggle with is to find a good way to display the comment in the table view. Selenium IDE displays the comment by default and hides the real command, so the command is only visible on mouse over. I find that very confusing. Better ideas are welcome!