It's hardly surprising that if the provider of the Operating System can no longer support it, then there's no means for the mobile network providers that offer that operating system to support it either.
There is however a way that you can run a completely new operating system on the HD2 without voiding the warranty.
The release of the latest official firmware from HTC for the HD2 on O2 back in February included the most up to date Radio ROM which is required to run Android from the microSD card. This means that you no longer have to install warranty-voiding firmware directly to the phone itself. You simply copy the Android folder of the installation that you want to a microSD card, then run it from within Windows Mobile. There are plenty of Android SD installations available from the lovely chaps at XDA-Developers.
There are some exceptionally fast and stable builds that are far more usable than Windows Mobile, and the Android Market and all Google accounts work perfectly. It's also possible to import all your Windows Mobile contacts and calendar etc from Outlook into your Google account and your phone. Couldn't be easier!