WARNING
Querying the TFS operational datastores directly is not recommended and the contents of these fields can change between releases and even hotfixes without notice. The official way to do this is to use the Client Object Model. Any changes you accidentally make to the data will leave your server in an unsupported state.
The TFS Client Object Model provides a relatively easy API to query build definitions. And from there you can use the standard .NET XML classes (either Linq-2-XML or a standard XPathNavigator) to quickly get the information you're after. The parameter you're looking for might even be stored in the build metadata.
This would result in the following code (as provided by OP):
TfsTeamProjectCollection server = new TfsTeamProjectCollection(new Uri("http://acmetfs:8080/tfs"));
server.EnsureAuthenticated();
IBuildServer build = (IBuildServer)server.GetService(typeof(IBuildServer));
IBuildDefinition buildDefinition = build.GetBuildDefinition("ACME", "ACME_v1.0.0");
object argumentValue;
if (WorkflowHelpers.DeserializeProcessParameters(buildDefinition.ProcessParameters).TryGetValue("SCA", out argumentValue))
{
Console.WriteLine(argumentValue);
}