So far, we haven't had to do that. I have added custom fields to the IS tables, added a tab to a form, and bound my controls on that tab to the dataset inside the IS form so when they saved, my data was saved right along with theirs.
On tabs I've added, they have been listings of related records. I have an add button above the grid and check to make sure the record state in the IS set isn't "new" or warn to save, then add the related record in my own plugin.
I haven't found a way to wire into their save method and insert additional updates within their transaction. I've limited my design to the two methods I stated above. I've suggested to Ramil that they add some eventing architecture to the system so we can subscribe to them and include ourselves in their transactions. Maybe someday...
Regards,