As of today the only way to integrate data into OPBCS is via a flat file. There is no direct integration into GL tables for an OPBCS instance. You can use EPMA automate and also you could seek to leverage REST API notably both require a flat file.
If your client does not want to drop a file then they will not able to get actuals into OPBCS . Not sure why they would be against dropping a file on their secure server and having a process run to load OPBCS.
Oracle has mentioned about having on premise FDMEE 18.104.22.168.X feed an OPBCS instance directly. Notably I, and other experts on the board may have more recent updates, have not heard any update about Data Management on OPBCS being able to connect to GL tables.
Your client can certainly file an Oracle SR for an enhancement request for the direct GL connection feature.
Hopefully the information was helpful and let us know how you make out.
There is no direct integration yet for the PBCS, but its worth while to check the REST API,
Also have a look at this (John had done a great job of getting that documented) http://john-goodwin.blogspot.qa/2015/09/planning-rest-api.html
All the best,