As someone who plays in VR, this was my first thought when I heard suggestion of space-legs being introduced. As you say, in the absence of a room-scale set-up or a consumer level VR treadmill, first-person locomotion is very troublesome in VR. Most developers opt for the rather unsatisfactory point-and-teleport method.
That said, one of the most satisfying and convincing mechanics that I’ve come across is Lone Echo/Echo Arena, where the player is in a zero-gravity environment and moves by grabbing scenery or utilising small propulsion packs on their wrists. It’s beautifully done, and having virtual hands really adds to the sense of reality (using the Occulus Touch). Of course, zero gravity is not what most people imagine when they think of space-legs, but it's the perfect conceit for VR (we are in space after all). I’m actually working towards developing a prototype that combines what I love about Elite with the freedom and sense of immersion that Lone Echo engenders, but I’m just an amateur who dabbles in development. It is certainly one of the things that FD will have to think very carefully about.
https://www.youtube.com/watch?v=bn1Lt1tdz4c