When I had my 6700, there was a way to still use the official ROM but trick it not to install the software.
-First you would do a hard reset
-Then when the ROM has finished installing the WinMo 6.1, Sprints updates will about to kick in, that's here you do a soft reset.
-When the soft reset has been performed, the Sprint updates (and maybe the HTC TF3D) will not be installed, you will be just using the core Windows Mobile.
-Then you would use Total Commander to browse the extended ROM files and you can then manually install the updates and software that you want. I'm not sure if this will work with the diamond, but I know that the Mogul still does the 3 second count down, and theoretically, this MAY work. There's nothing to lose, all you would have to do is just reflash if it doesn't work.
Here's the link to the instructions on how to manually install the Sprint ROM on the 6700:
http://ppc6700users.com/forums/viewtopic.php?t=8
Let me know if this is helpful.