Yeah, I also hate that comments are not synced with cross-docs.
What you can do is disable the native comments and build your own commenting system with one/several tables, that you can make visible in all the relevant detail views and keep them synced across documents.
For example, in this post I created a simple commenting system
It will never be so nicely integrated as the native solution, but you have the power of coda to customize it and taylor it to your needs just like any other table.
Hope this helps,
Pablo