Nope, in all seriousness what is the issue with making a PA prior to doing an engine run up? I wouldn't bother explaining about flap selections, but I will always try to make a PA to the passengers explaining why I am increasing the trust against the brakes before letting go and launching down the runway.
It's not hard. Those that know me on here will know that I am strongly in favour of keeping the passengers informed of what's going on during their flight, and the number of complimentary comments I get when passengers disembark reassures me that it is appreciated.