You can change the centreline to any symbol type in the Visualization tab on the part settings.
I picked this up and use it regularily.
http://aclabels.net/index.php/products/mep-label-for-archicad?i=1
For ductwork I set the ID as the trunk or branch name and the MEP Duct label will display the ID and the size. What is great is that when the duct or pipe size changes the label automatically updates. It saves tons of time rather than having to manually search and update the plan.
Intel i7-6700@3.4GHz 16g
GeForce GTX 745 4g HP Pavilion 25xw
Windows 10 Archicad 26 USA Full