I love this idea, though I do wish there was a way to have the wood extend further (i.e. covering the wood side even when extended).
My two concerns- beyond stability, which you obviously know about- are heat transfer between components and access to air. Some sort of system to allow for dividers between components could be key- being able to have a separate GPU "compartment", for example. Obviously that can't be a solid wall, but some sort of a divider that allows the riser and a power cable through but otherwise blocks CPU/GPU air transfer seems important. The other piece is what happens to the air- it seems like positive pressure is the way to go here, with fans sucking in, but whether there's enough ventilation with only the side and not the top/bottom open is an interesting question.