This is by design, if you want to assign the id manually, then the generator is "none" (meaning you don't want Aurelius to generate it for you). The only downside is that Aurelius won't know if the object is must be inserted (a new one) or updated (an existing one) but you would have the same problem anyway if you were able to manually assign a value to a GUID generator.
A little modify to your source could allow more flexibility:
We will have to review it better and think what is the best way to do it (according to our guidelines). Personally, I don't like default parameters in such high level, high used methods. We might probably end up with a different method name.
Many thanks for your answer.
As we do data migration between database as well, we would require this functionality too. Any progress on this? Maybe an extra attribute for allowing identity-inserts?
For data migration users usually build a different mapping setup where they manually change the generator for the column at runtime. But yes, we should consider something like that. May I suggest that you add a feature request so it can be voted?