Connect Android device as wifi adapter to a PC to create an ad-hoc network Sometimes when being at a friends house there is no wifi but wired internet access. 

In these cases it would be extremely useful if I could plug my phone in and create a wifi network just like when providing a wifi hotspot except that this time not the carriers data plan is used but the already available network access on the "host-PC".

Does anyone know if and how this is possible?
This appears to be a little over "USB Reverse-Tether" requirements :-)
You actually want to share your wired internet backwards over the WiFi of your android phone.
The reverse-tether, imo, ends at the point of being able to use your wired Internet over USB from your android phone. Now, pushing that over the phone's WiFi to other WiFi devices sounds like one step more.  

I am not sure I have hard this done yet -- but, I'll certainly be happy to see this work.  

The HTC feature of Internet pass-through for USB would do the reverse-tether.
Now, the next step is to make the phone 'bridge' that to its WiFi and allow others to connect through it.

Another support in some devices is when you can make your phone a 'hot-spot' to share its 3G/GPRS access over the WiFi. So, in theory, there has to be a way to reverse-tether the phone and then share that access over WiFi... but, I don't know how yet.  

Hope, this answer helps others to focus on detail of this requirement. 

As an alternative, you could use a USB WiFi dongle on the PC.
However, I understand your interest in getting a solution that will work with an android phone rather than carry a USB device for the purpose.