My main issue with this sort of thing seems to be that if it gets too hot, no matter how much liquids I take on I end up feeling very ropey after the ride, dry mouth, unsettled stomach, liquids just passing through rather than helping. I figure this is due to loss of electrolytes and stuff like that but admittedly this is not a field I know much about so perhaps I'm reading the signs all wrong.
Sounds like you're getting to a point where electrolyte imbalance is causing problems and if that's the case then drinking water isn't going to help much as it'll just make you feel full without actually replenishing lost electrolytes. If you're using energy drinks at this point, hyper- or hypo-tonic fluids will just lead to greater 'turnover' of water which may explain your feeling of water passing straight through.
How come you're getting so dehydrated? Dressed too warmly? Or is it just that you drink hardly anything on the bike? Try something like the SIS Electrolyte drinks or just experiment with making your own: fruit juice of choice, water, sugar, salt and a bit of orange juice, mix to taste (about 50:50 water/juice and a teaspoon of salt, tablespoon of sugar and a cup of OJ in a 3L camelbak.