Evening, Well from my perspective, I would agree with you that 2 axis movement would be needed in order to able to track in a rotational environment. That said, and I don't read Japanese, it appears that the K5 does have some limited rotational capability on the sensor. Gimbal posted this link...
... that tends to indicate that the K5 does have the necessary 2D capability. I wish Pentax would strike while the fire is hot and release the manual, and as much engineering data that would help the astro folks understand the capabilities along with the limitations of the unit. Obviously, there is a hard limit to the shutter time which is linked to the range of movement of the sensor. Up to 300 seconds is a pretty long duration. Running back to back frames and stacking, provides a capability that currently really does not exist out side of having a tracking mount.
The k5 body at say ISO 80 (or even maxing it out to what? 52,000), with a really sharp lens like say the 31 ltd or maybe the 14, for a wide field view, should potentially be able to produce some pretty stunning imagery. Using the sensor's dynamic range, and ability to pull detail out of shadows, should put astro work in just about anyone's hands (with out going overboard with some special tracking mounts and setup).
I have the K20 and not the K5, so the sensor's range of movement described within the K5's manual would be important here.
One item that I noticed, when you image something with stationary objects (landscape), its the landscape that gets smeared. Take a look at the images here that has some landscape elements in them... (again supplied by gimbal)..