Hi there, fellow Ergotron arm users, I´m a freelance illustrator and inventor of things.
This is not a question because I know the 13HD Cintiq as also the Companion doesn´t have a VESA mount.
This is about "how" I have made it possible to make smaller pen displays usable with the Ergotron LX arm.
For anyone considering such a setup, it does work and is pretty sturdy, versatile as it allows to have the tablet on a workstation and ready for mobile use which is not possible with the heavier, clumsy ones.
For more information please visit my blog as it gathers 2 posts regarding this topic, frequently asked questions, reader comments and more:
Post #1: http://fantasiox.blogspot.de/2013/05/setting-up-wacom-cintiq-13hd-with.html
Post #2: http://fantasiox.blogspot.de/2013/09/cintiq-13hd-and-ergotron-arm-few-month.html