Correct, WIA is only for reporting the inbound/outbound stock movements.
I do not know how well your plants abroad scenario will work with sales to end customers though.
I hope that you are working with a legal consultant on the project to advise on topics such as whether any of the two countries has requirement for specific number invoice range without gaps for example, additional country-specific reporting requirements etc.
When you finally get it working, please post a document in SCN, so that the members of the community can also benefit from your experience.