Check out VoiceControl. http://www.mobilevoicecontrol.com/
You should be able to accomplish your mission somewhat... it won't be a recording of the dictation but they can voice command a new email and you'll always have the transcriopt as record.
BTW.. it has the BB 8700 listed on the home page... but then goes on to list iDen as a system req. (Sprint/Nextel) so this may not work