Just to make sure you know about the requirement:

Multiple people are linked to a centralized billing entity.  They want one bill but internally allocate the cost to the various cost centers/people/budgets etc.  So the comment could be used for:

A cost center or the date of service or person who ordered it or any other 'identifying mark'.  So we need to capture this, like the reference field on an invoice / sales order, but at the line item level.  Maybe a reference field would be a better name for it, bit comment + reference at the line item would probably solve many other potential uses.

Thanks

This is because you cannot change the item description field.  It is driven from the Item's description.