You would have to account for vibrations from many sources that could cause the spinner end of the extension tube to touch the rotating assembly.
I'm guessing that muzzle deflection due to firing the MK 103 (barrel harmonics), would be greater than deflections due to all other sources at that location. If it was me, I'd just add a bushing or bearing to constrain the muzzle, and see how much it degraded accuracy. But that's just a shot in the dark on my part. You've actually seen the 335 drawings.
Is it possible from responces to this thread that HiTech is being genrous to us with convergence settings available to nose mounted guns?
I think you're right. According to Stoney (and his data is usually very accurate), the centerline mounted 108's were non-adjustable. I'd assume the spinner-mounted Mk 103 on the Do 335 would be installed the same way. So yeah, convergence should probably be locked. In fact, HTC should probably just remove the Mk 108 entirely from the german fighters.

(Can you tell I've had my share of "tater enema's" courtesy of the Muppets?)