This is not entirely accurate, but it should be a pretty good ball park to create a report using "Item Centre Line Length" & "Item Connector Effective Size #2" in a calculated field.

The center line length will let you basically evaluate the developed length of an elbow as a cylinder and any other part that has a length. The effective diameter is set to connector 2 because elbows don't actually use connector 1 at all. This will mean that you would capture the smaller side of reducers rather than the larger side, you will not capture the branches of tees and caps would never be seen at all, but I would think that is better than not capturing the surface area of elbows.

This is long and messy, but I think this will get you close to what your wanting:

2 * 3.14159265 * (Item Connector Effective Size #1 / 2.0) * Item Centre Line Length  + 2 * 3.14159265 * ( (Item Connector Effective Size #1 / 2.0) *  (Item Connector Effective Size #1 / 2.0) )

Like I said, its not perfect, but you could easily work out your own +% safety factor to cover the losses.