I have had an HTC Mozart locked on Orange UK for a couple of years now. I also have a spare lying around that I tested this on. Neither have had a an update since Mango (7.10.7740). This is because Orange never approved any updates after Mango, which was so nice of them. Checking for updates via Zune irritatingly says that your phone is up to date, when it is far from up it, there are at least 2 updates since this:
However, it is possible to debrand the device, which removes the Orange customisations and branding from the phone, and although it will still be SIM locked to Orange, other than it turns it into a generic HTC Mozart, and will get any update from Microsoft/HTC immediately. So doing this should also mean that you will get Windows Phone 7.8 as soon as it is released, not if and when Orange decide to release it.
The downside is that you will have to reset the device, so you will loose any customisations, game saves etc. To upgrade, go to http://forum.xda-developers.com/showthread.php?t=1002391 and follow the instructions. Make sure you carefully follow the part where you extra the .exe using 7zip. You need to have a working branded ROM on the phone before you do that, don’t skip any steps.
Use the ROM versions exactly as mentioned in the post (assumes you are on Orange UK). Once you install the second ROM, plug into Zune and let it update. This will take several hours, whilst it installs many updates (I am not joking!).
Finally it culminates in this as you get the Tango update, and internet sharing!
This should work with other Mozart branded devices, but your mileage may vary.