Set update policy on the runelite repo directly. It appears this doesn't work on releases though. Use developer-mode switch.

src/main/java/net/runelite/client/ArtifactResolver.java

@@ -60,7 +60,6 @@ public class ArtifactResolver
 
 		LocalRepository localRepo = new LocalRepository(repositoryCache.getAbsolutePath());
 		session.setLocalRepositoryManager(system.newLocalRepositoryManager(session, localRepo));
-		session.setUpdatePolicy(RepositoryPolicy.UPDATE_POLICY_ALWAYS); // this causes a refetch of maven-metadata.xml, but not any binaries
 
 		//session.setTransferListener(new ConsoleTransferListener());
 		//session.setRepositoryListener(new ConsoleRepositoryListener());
@@ -99,7 +98,9 @@ public class ArtifactResolver
 
 	public RemoteRepository newRuneliteRepository()
 	{
-		return new RemoteRepository.Builder("runelite", "default", "http://repo.runelite.net/").build();
+		return new RemoteRepository.Builder("runelite", "default", "http://repo.runelite.net/")
+			.setPolicy(new RepositoryPolicy(true, RepositoryPolicy.UPDATE_POLICY_ALWAYS, RepositoryPolicy.CHECKSUM_POLICY_FAIL))
+			.build();
 	}
 
 	private void validate(List<ArtifactResult> artifacts)

src/main/java/net/runelite/client/ClientLoader.java

@@ -15,9 +15,13 @@ import org.eclipse.aether.artifact.Artifact;
 import org.eclipse.aether.artifact.DefaultArtifact;
 import org.eclipse.aether.resolution.ArtifactResult;
 import org.eclipse.aether.resolution.DependencyResolutionException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class ClientLoader
 {
+	private static final Logger logger = LoggerFactory.getLogger(ClientLoader.class);
+
 	private static final String CLIENT_ARTIFACT_URL = "https://static.runelite.net/client.json";
 	
 	public Applet load() throws MalformedURLException, ClassNotFoundException, IOException, InstantiationException, IllegalAccessException, DependencyResolutionException
@@ -26,9 +30,18 @@ public class ClientLoader
 
 		config.fetch();
 
-		// the only thing resolved here is the injected client, which has no dependencies, so no need to switch developer mode
-		ArtifactResolver resolver = new ArtifactResolver(RuneLite.REPO_DIR);
-		resolver.addRepositories();
+		ArtifactResolver resolver;
+		if (!RuneLite.getOptions().has("developer-mode"))
+		{
+			resolver = new ArtifactResolver(RuneLite.REPO_DIR);
+			resolver.addRepositories();
+		}
+		else
+		{
+			resolver = new ArtifactResolver(new File(System.getProperty("user.home"), ".m2/repository"));
+
+			logger.info("In developer mode, not fetching updates for client");
+		}
 
 		List<ArtifactResult> results = resolver.resolveArtifacts(getClientArtifact());
 		File client = results.get(0).getArtifact().getFile();

src/main/java/net/runelite/client/RuneLite.java

@@ -14,7 +14,7 @@ public class RuneLite
 	public static final File RUNELITE_DIR = new File(System.getProperty("user.home"), ".runelite");
 	public static final File REPO_DIR = new File(RUNELITE_DIR, "repository");
 
-	public static OptionSet options;
+	private static OptionSet options;
 	private static Client client;
 	private static RuneLite runelite;
 
@@ -67,4 +67,9 @@ public class RuneLite
 	{
 		return renderer;
 	}
+
+	public static OptionSet getOptions()
+	{
+		return options;
+	}
 }