Is it possible to co-locate at rackshack?

The short answer for the moment is No.

There has been a thread which indicated that RS is seriously looking at colocation. Currently they are moving into a new datacenter, adding new server types and are backed up on customer upgrades for memory, hard drives, etc.

I expect they will ultimately add colocation as part of their services. It is a logical extension. That being said, I think it is some months down the road since their plate is currently very full. Their recent initiatives will grow their business and keep them busy for the forseeable future without branching out to colocation.