Well - Chris77, as I know the RZ 67 as
a very stable, solide camera I would trust this system from it's weight - you discribed.
I personaly use a P67 and this camera is also quite stabile.
We have 2 bajonet systems with Pentax
(if you may also know) - one to smal lenses and the other system is used with
big lenses.
I have a 300mm/4 ( not so heavy )...?
I always thought - the camera is build to this!
Mounted on a tripod I often have it used.
Now I bought a 2x extender and I wonder a bit about the weight.
I is also a case of lever forces.
So I tapped at the tripod mounted system with my fingers.
Yes it is stable enough - but I will never
mount this system on a tripod again because I can broke the camera bottom very easy
with not so much power.
The 300/4 with extender is like a crowbar to the P67.
Pentax 400/4 has only a little much more weight but the lens has extentions in the middle to mount the system on a tripod.
Very late I realised : The second bajonet system from Pentax to heavy lenses is more
stable (of cause it is so) but this is just
to manage the weight of the camera.
It is not in concern of the much more lens weight.
In short - beware of with heavy lens systems on a tripod.
Some told me Rz67 is heavy as a tank.
Perhaps I mixed it up ( they may mentioned this in concern to RB67 ).
But in reality it is NO TANK
...
It is just a Mamiya.......
with regards