Credits to all who have answered, I've tested all your proposition and here's what I've found , as of 2015:
There is currently no free AND reliable solution to read and manipulate .dwg and .dxf files. This applies especially to latest versions of AutoCAD, ex : Autocad 2010, Autocad 2013 etc, RF 2013, RF 2010.
Reliable (and expensive) solutions would be Teigha by ODA (2000$ a year) and RealDWG in C# by Autocad (5000$ a year)...
Cheaper alternatives are as such :
- AutoCAD I/O for 10$ per month (as suggested by Augusto Gonclaves), but limited functionalities for now.
- writing LISPS routine and running them with a SCRIPT file in a paid instance of AutoCAD 2015/2016..
- wait for a more complete implementation of AutoCad Core Console..
Open-source libraries such as Kabeja and YCad may be able to deal with older AutoCAD versions but do not expect much from them as these projects are inactive since a few years ago. It's worth to keep an eye out for Apache Tika though, since they started writing parsers for .dwg files but development has been slow.
On a personal note, I think that AutoDesk has a strong and selfish monopoly on the CAD market now and they are making it difficult for others to join in. The format's version of .dwg files changes so often and I guess open-source developers will have to give up at some point and use the official product instead.
Let's hope for more open-source solutions in the coming years