"mockup" object to simulate HMD

The SDK offers a debug device, but it is fairly limited. You can't do anything to simulate sensor inputs or anything like that.

This looks like a good solution, however, it seems it's only available in the 0.3 SDK, not 0.25?

I don't know much about the older SDK, but it does appear that there is no API for debug devices in it.

I would advise against using the older SDK since it doesn't support DK2.

Now, take what I'm about to say with a grain of salt because I haven't actually gotten by DK2 yet so I'm not fully comfortable with how the integration works...
But what you could do is design a simple little android/iOS app that gets the framebuffer of your computer, while simultaneously sending the gyroscope data back to unity via OSC (search Ventuz.dll OSC for the package you'll need). You won't have 6DOF, just the rotation, but it should work pretty well.

As a developer in a OpenSource project*, I am in the same situation. One of the team does have access to a borrowed DK1 but he's on the other side of the world.

I found this project, which fakes being the sensor pack in the DK1 using a easily available $15 STM32F3 discovery board
https://github.com/yetifrisstlama/Foculus_Rift_Tracker_STM32F3DISCOVERY
http://yetifrisstlama.blogspot.ca/2014/03/the-foculus-rift-part-2-reverse.html

Don't expect it to perform as well as/exactly as the real Rift, but it's good enough for quick testing. It works with 0.2.5 and 0.3.2 SDK's.

You _really_ want to prove your HUD in a real Rift as subtle mistakes will not be as obvious on a PC monitor.

Cheers,
Simon.

* http://speed-dreams.org