Did you know that WoW includes a raid browser to assist with finding or forming groups? Don't worry, neither do most people. But it exists, and as we work towards revamping and expanding this feature for Warlords, we figured that we could also improve the usefulness of the existing functionality in the short-term.
If you click the "Other Raids" button in the social Raid tab, you'll see a list of raids. For 5.4.2, we've added two new categories, Mists of Pandaria Raid (Flex) and World Boss, which are specially enabled to function cross-realm. If you're looking to join a group for these activities, you can check off the appropriate fields and click "List My Name" below, and you'll put yourself into the LFG pool for those activities. For these specific activities, this pool will function cross-realm.
If you click over to the "Browse" tab on the right side of the screen, and select one of these activities from the drop-down list, you'll see a list of players across all servers who are looking to join groups for that activity.
Please give this a try on the PTR, and report any feedback or bugs you encounter. Thanks!