As far as I know, there is no way to monitor oil pressure or intake air temperature. I think there are OBD2 bluetooth devices that allow you to use your phone to see such data.
The other useful thing about the 753 hack is allowing you to use a rearview camera if it is hooked up. I bought a cheap camera on ebay along with a Fakra connector to hook up to the back of the nav unit. The 753 hack allows you to use access the camera.
The only other Easter egg I know of is the 1791 hack that allows you to use navigation on the move. I don't remember offhand how to access it. I believe it doesn't work on newer model LR3's. It used to work on my '05, but then I updated the navigation disc and lost the ability to access the hack.
Come to think of it, there is one other hack that I saw on the disco3 site. It puts the nav unit into a Range Rover mode and allows you to access the rearview camera automatically when the vehicle reverses. There are a number of problems with this. First, it requires modifying the wiring on the connectors to the nav unit (if you attempt this, DO NOT cut the wire on your connector as most people have done. I have found that the wire can be carefully coaxed out. If you decide to undo the mod, you can put the wire back in). Second, the nav unit loses the ability to auto dim. It stays way too dim during the day, so that it is hard to see the display. There is a way to overcome this by soldering in a diode on the circuit board. Third, the hard buttons are swapped in functionality and you have all sorts of screen buttons that are not applicable to the LR3, for example radio controls. Lastly, I believe the nav firmware needs to be up to date, which requires something like a Faultmate.