This is a feature that many people have asked for: the ability to use grid mode with ModreX. We have now made this simple test server for testing interoperability of ModreX regions in an OpenSim grid.
Getting the region in ModreX mode is a pretty straightforward procedure. First configure your region according to OSGrid instructions, then modify ModreX configurations. First thing to do is to enable the default Event Queue and disable RexEventQueue. Without this, once entered to ModreX region, one can't teleport or walk out of it. Second thing to set is the client stack plugin. Set clientstack_plugin configuration to "ModularRex.dll" instead of the default LL. This makes UDP listener spawn RexClientViews instead of LLClientViews. This is important when using realXtend features like meshes, particle scripts and so on.
Final thing to notice from ModreX options is the ClientView option. By default this option is in mode compatible. The compatible mode is ment for interoperability purposes, it supports both LL and realXtend clients. However there is a drawback, compatible mode doesn't support smaller avatar update messages. This means that clients spend twice as much bytes on avatar update messages in compatible mode than in Naali or 0.4 mode. Both Naali and 0.4 mode do support these smaller avatar update messages, but SL client crashes on these messages, so these modes shouldn't be used in grids where SL clients are also used. Differences with Naali and 0.4 modes aren't that great, but as you can expect 0.4 supports the older realXtend clients and their features and is also compatible with Naali clients. Naali mode contains a bit more experimental packets that aren't handled properly in realXtend 0.4 series clients.
Now that the region is configured it can be added to the grid. Although you can see from the picture that the region is working in the grid it is still lacking some features. User needs to press Shift+R (on the 0.42 realXtend viewer) to switch to OGRE rendering and to view the scene properly. Currently grid mode doesn't support realXtend authentication or realXtend avatars. Also at time of the testing there were some problems uploading textures and the physics engine crashed few times. Mediaurls are disabled from the client since it didn't receive rex_mode flag in login response.