You should find you have these 3 fields available to you on sc_req_item. Each contains all work notes, comments, or worknotes and comments in one thing.
Your issue is that you want the act of commenting to be a trigger, and you can't know when that was done if other activity is happening on the item (except at the exact time it happens).
The options I've used before:
. A metric - you can create a metric every time a comment is added. You can then report on these. You can get them in one pane of view if you have a database view for sc_req_item, metric definition and metric instance. Can't remember if you can do this with just a metric that runs on field duration, with the field being comments? In the past I've done it with a business rule, and inserted a metric via the business rule (as in a business rule we can determine exactly when a comment is being made and if it was the system or not).
. A new field for "last customer update" or something of that like, as a date time field. I've seen this done as well, you'd need a new field which would basically be a "last update by a human" equivalent to sys_updated_on. You could then populate this every time a comment, or worknote is added using a business rule (condition would be when comments changes). You could cater this to your needs with a filter on the user doing the update, i.e filtering IN or OUT the data depending on whether they are or aren't your common company user ID (you could know a member of the service desk has made an update because comments changes AND updated by is fred@servicedeskcompany.co.uk).
Final option is you've got the "sys_journal_field" table. Can't remember if as an admin you can report on that, think you can but you might have issues sharing it with normal users. Runs dead slow though and I believe reporting on is is not recommended. (this is where the individual comments are actually stored)
