I would STRONGLY advise you against this.
HTML email was not enabled in SP5 for a reason. If you hacked it and enabled HTML mail capabilities, then you are currently running an unsupported BES in a large production environment... and that's not wise.
Wait until RIM releases a version of BES with official HTML mail support, as well as an official OS 4.5 for your handhelds.
I think OS 4.5 is available for the 8310 now... but not from Rogers.
Your mass firmware update for 400 phones is not going to be easy, no matter how you decide to tackle it.