Alan, this Autodesk University class from 2013 has some good info on editing these files. Good info on other things, too.
I wanted to say that this was a huge help.
I was able to create several custom fields, and they work quite well. If only I could figure out how to be able to edit the custom fields in the structure properties and it not break the link in the swap part menu, which also screws up the style when you dref the network out.
It seems like you can create a new part in your part list and change the properties there and it will hold, but that just isn't really practical. My other thought is to just break up each structure type as it's own family, that way, when the link is broken and it reverts to the first item in the list, it will only have 1 item in the list to choose from. I haven't tested to see if this will still display correctly with drefs, but it's worth a shot.
All I wanted was a way to add a little piece of extra text to the structure that would show up anywhere I dref'd the network.
I stumbled across this, which basically explains what Jeff's video does, but it also shows the custom property actually being added in part builder. Not sure I see the point, but there must be one.
http://beingcivil.typepad.com/my_weblog/2010/07/assigning-optional-properties-to-a-part-size.html