I think the minute silence (for all cyclists who have died on our roads in the last year) is actually quite powerful in this instance - out in the open (i.e. not confied within a stadium) it's weird the hush that descends, and even pedestrians walking by stopped talking. I lead it last year (had a better whistle ;) ) and it felt quite emotional - but there was the same issue of the message not getting down the line.
It is very very easily rectified in one of two ways, alreadty mentioned in this thread. Both need perhaps a little more publicity of the silence before it takes place.
1. People with whistles strategically located down the line. Would take about 15 seconds for the message to get all the way.
2. The balloons idea - I like this quite a lot, but probably needs even more publicity beforehand than a whistle being blown.
That's really it, doesn't need to be overcomplicated with extra walkie-talkies or so on. 6 degrees of separation probably means three marshalls walking the line 10 minutes before the silence will come on someone they know willing to blow a whistle every 50 yards or so, and while handing it over reminding the cyclists around there about the silence. Those marshalls then take up the end position ensuring the silence is knwon right to the very end, and they're in place for marshalling the back of the ride.
(and the silence is even more powerful when backed up by that bell ringing afterwards, the contrast makes it so potent - I'm not sure just bell ringing would have quite the same message).
Just my tuppenceworth.