As already perfectly explained by ChssPly76, Hibernate's proxies don't help with unconstrained (nullable) one-to-one associations, BUT there is a trick explained here to avoid to set up instrumentation. The idea is to fool Hibernate that the entity class which we want to use has been already instrumented: you instrument it manually in the source code. It's easy! I've implemented it with CGLib as bytecode provider and it works (ensure that you configure lazy="no-proxy" and fetch="select", not "join", in your HBM).
I think this is a good alternative to real (I mean automatic) instrumentation when you have just one one-to-one nullable relation that you want to make lazy. The main drawback is that the solution depends on the bytecode provider you are using, so comment your class accurately because you could have to change the bytecode provider in the future; of course, you are also modifying your model bean for a technical reason and this is not fine.