BCS in sharePoint Online

Hi,
I want to create an App on SharePoint Online Office 365 E3, may be using Infopath or by custom development.
But the final values of data from sharepoint are to be pushed into dynamics Axapta ERP,then inside ERP, these values get Approve/Reject status,this value again need to be pushed back into sharepoint 365 library.

How BCS is suitable for this scenario, because BCS works on External list only, so IP Form Library will  help/Not help ?
http://msdn.microsoft.com/en-gb/library/jj163782.aspx
Pls Suggest