Thanks to the article posted by Jensd (http://knowledgebase.progress.com/articles/Article/What-are-the-basic-steps-to-authenticate-REST-clients-against-the-OpenEdge-database-User-table) Following the steps there allowed me to create a custom class, which then runs as part of the spring authentication.
Implementing the appSecurity-form-oerealm option for Progress, under the OERealmUserDetails section has an XML property node with a name of "realmClass". In this property you specify the name of your custom class, which tells the spring framework to run this to validate credentials.
A ValidateUser method and ValidatePassword method must be in this class. These methods allow you to run any OE code you like to access your database and validate user credentials.
*Note that this method of authentication is for older "Classic" AppServers, and an older version of spring. Progress have now released PAS for OpenEdge which works differently.