Can data be replicated from a MySQL slave back to a master without the slave containing all the master data? We have a PHP/Symfony 2 application using MySQL as the database. The database contains a large amount of data.

The application is deployed to a separate server(s) and an empty/clean version of the database deployed. The application is then run on a local network as the application is used where internet access is not always available (hospitals, banks etc).

Once the application has collated all the information are there any solutions for replaying the data back to the original database?

We are not using Master/Slave as we do not want the Slaves to contain all data from the original database.
When using master/slave you do not need to replicate the all instance.
You can go up the the table with the replicate-wild-do-table parameter.

May be this could help you in what you are trying to do