Are there techniques to graft hinoki onto old wood?
It’s the same process as grafting juniper. If you’re looking to do side-veneer grafts, late winter is likely the best time to do it.
Feel free to post a pic of what you’re looking to graft if you have specific questions about how to go about it.
Fun tree - I can see why you’d want to graft it.
The main thing with grafting a tree like this is that you graft into locations, whether on the trunk or the branches, where there is good sap flow. Up high on the trunk may be difficult because the apex is gone, but I’d expect grafts on the branches or lower on the trunk to do well.