Sounds like you need to use slmgr as a run once to change your key once the image is deployed and use the mak for the image.
We use MDT build our base image with driver injection and certain apps are automated into that build. The build and capture is automatic so it just gets the latest updates. We also use app-v which helps greatly with a great number of apps.
Just incase you've not seen it, SCCM 2012 does offline wim servicing:
Ronni Pedersen's deployment ramblings » ConfigMgr 2012 – Offline Service for OS Images
I read your original post wrong... Try this... Find the appropriate key from the link below and change both the key in your VM install, and in the answer file to match. When using one of these keys Windows will try to activate against a KMS first thing.
KMS Client Setup Keys
If I had to guess, your image right now is trying to activate directly with Microsoft with some key and completely bypassing your KMS server.
I have to agree, three days to build an image is too short. I just activate using a MAK key.
On a similar topic, it looks like Windows 7 SP2 is predicted for a early-mid 2013 release. Does Microsoft expect everyone to deploy SP2 on existing installations or rather the quicker/easier way to create a newer image with SP2 integrated? And more to the point, how is this going to work with any KMS or MAK activation system?