Best area to add these is in the InitClass* methods, like the InitClassEmpty method and then do that via an include template, bound to Custom_EntityInitializationTemplate. See 'Using the generated code -> Adding your own code to the generated classes'.
Code in a template bound to Custom_EntityInitializationTemplate, will end up right below the code in InitClassEmpty (adapter).
You have to take some measurements to be sure you set the values only when the entity is new. Please see this thread for ideas: http://www.llblgen.com/tinyforum/Messages.aspx?ThreadID=3194
As mentioned in that thread, some rescheduling in the runtime lib has been done at may 20th, so the init code indeed can figure out if it's handling an empty fields object or a filled one.
Be aware that setting fields makes them 'dirty' and if the entity is added to a graph, will make the end up in the database when the graph is recursively saved.