The biggest problem you will face with the rotatable cage is initial allignment. If the center of the mirror and the "center" of the diagonal are not Perfectly centerd in the tube, you will have nightmares everytime you rotate the cage with the collimation. I've seen few scopes that have these type cages that will maintain collimation.
I've given this a lot of thought simply because it seems to be a very ideal solution to viewing position with any type Newtonian and have about decided that the safest bet is to make the entire tube "rotatable". Even with a Dobsonian mount, this should only necessitate the mirror end of the tube assembly being round (tubular). One could then fit it into a "socket" on the mount that has teflon bushings. The Socket would carry the trunions. The result would be a tube assembly that could be fitted into either the Dob mount for portability or into a permanent mount for equitorial tracking.
Note that this design does not rule out truss tube structure...only the change from square mirror end to a tube type mirror end.
(Just my 2-cents worth)
Mike
* RM 1.3 * Lunatic asylum: where optimism most flourishes.