What do you think about having text lines in SO and invoices?
I'd appreciate a separate item type = textblock / comment.

The identifier would be an option field defining the item as text. Like being a service item. 
And when this item is selected in the document lines, actually no values will be printed on the report but just the text of the description.

It would be great to have that and I guess it's not a too big project to realize, what do you mean?

Have a good week,
Gabriel