I have a solid piece of geometry (a link) with a pin hole. How can I specify a pin joint in the centre of this hole where there is no node?
I suggest you follow these steps (see also screen shots attached):
Assemble > RBE2 > Mid location>select all 6 DOF & the inner surface.
This will create a node in the centre automatically which you can then constrain as you please (see 2nd image).