We had a similar situation where a variable on the API had to set differently for different users. We did the following:
-Created a custom serviceImpl which extended OAuth10aServiceImpl (may be OAuth20ServiceImpl in your case).
-gave it a method to set the variable on it's api class
-after service is created by your ServiceBuilder lookup the appropriate value and call the setter method of the service.
-continue with normal OAUth token flow
Note that you also need to let the API know to use the custom service class, for example:
@Override
OAuthService createService(OAuthConfig config)
{
return new CustomServiceImpl(this,config)
}
Hope that helps