Totally agree about strong typing. Just do this, I think it's simpler and I don't see where you'd need casting.

public class PTableStatsImpl implements PTableStats {

    // The map for guide posts should be immutable. We only take the current snapshot from outside
    // method call, store it in this atomic reference to prevent race condition.
    private Map<String, byte[][]> regionGuidePosts;

    public PTableStatsImpl() {
    }

    public PTableStatsImpl(Map<String, byte[][]> stats) {
        regionGuidePosts = ImmutableMap.copyOf(stats));
    }

    @Override
    public byte[][] getRegionGuidePost(HRegionInfo region) {
        return regionGuidePosts.get(region.getRegionNameAsString());
    }