Hi Ian!
Assuming there aren't a lot of hills, I don't think a heavy bike will be much of a detriment to you on a Century ride. Once up to speed they just roll along if you do. Weight is a factor in hills and if you accelerate/start/stop frequently, moreso if the wheels are real heavy. My Nomad weighs 20kg dry, and I find it no trouble to do regular 200km/125mi rides on it, though my times aren't as good because it tempts me to go off on gravel side roads and dirt. If I stay to pavement, then my hundred-mile time is pretty consistent at around 5 hours 50 minutes in mixed terrain if I put my head down and go and don't stop.
My rando bikes are 14.5kg dry and they're what I have and use for 200, 300, and 400km day rides including in sustained climbs (there is one long 5% grade for 53 miles going up to Santiam Pass in the Cascades and it is a haul). The rando bikes are more pavement oriented (32-34mm tires, either Bontrager road slicks or Paselas) and lighter, so I tend to take them for the really long rides which are also on pavement. If there's much rough stuff (heavy rock ballast, logging roads) then I take the Nomad with its 26x2.0 Duremes. The thing that makes the choice is not so much the weight or distance, but the surface.
On both bikes, on day rides in summer here I usually carry my rack pack with a wind jacket, and perhaps my small spirit stove kit and some energy bars, and each bike has minimal tools in an underseat bag along with patch kits and a spare tube. I typically carry about 3 liters of water with me and refill completely if it is over 37°C.
I'm not far off your weight at 78kg and have my pie packed on to the "corners" near my kidneys. As to making the bike lighter, I think I'd look at what I really needed. Water weight is my biggest offender, followed by food and some means to prepare it, then clothing. My complete rain gear weighs 1kg (Generously vented waterproof jacket with hood, waterproof/breathable helmet cover, Gore-Tex pants, neoprene booties, and Gore-Tex gloves), so that stays home if is is to be truly dry for the duration, but sometimes the weather is drastically different 50-100 miles away and at altitude, so I often pack it anyway. If it is a charity ride and your bike is already in good shape, you'll not need many tools. If there are water stations, you can scrimp there. Otherwise, I don't think you can do much to lighten your present bike enough to make a difference. If you were to change to a lightweight road or Audax/randonneur bike, yes - you'd definitely notice a difference, but I can't imagine the extra weight of your bike being the deciding factor in doing the Hundred or not.
I agree with everything Brummie wrote: Fitness, comfort, and pacing are really more important. All the points made by others are sound as well.
In DanneauxWorld, a 16kg bike is not bad for putting in the bigger miles.
All best wishes to you on your effort, Ian. For charity!
Dan.