I have done coding using the ProSteel API and it is pretty extensive and flexible. I made improvements to the logic used to add position numbers on the steel structure objects as well as creating a two way sync of object properties between AutoCAD and a database. I also created other customizations to automate the workflow further both for working with the 3D model as well as the 2D generation. Another thing I did was to manipulate the ProSteel model so it worked better to use the information in Navisworks.
Either ps3d_com.arx (PSComWrapper Type Library) or ProStructuresLoader.arx are used and it works fine to develop with either AutoLISP or VBA. (VB.NET or C# should be possible to use)
Unfortunately there are not many resources about usage of the ProSteel COM interface other than the included documentation but the other day I run into this site that has some examples posted.
ProSteel was created by Kiwi Software GmbH and later acquired by Bentley and is an add-on running on the AutoCAD platform.
If you need any customization done for ProSteel (or AutoPLANT Structural) feel free to contact me.
Previous blog post talking about ProSteel 3D.