The question about a compost toilet flush is about how well it absorbs odours and has the capacity to stop flies and midges.
Coir is too light and because of this it is too airey and lets out odours.
Because it is too light, even when you have added water to it, it lets midges get into the waste…if not flies.
HH-5 is however a far superior product because it is more dense and is at the right moisture content to absorb odours and stop flies etc., It has to be damp in order to activate the organisms within it that start off the composting process.
Coir on the other hand is too dry and has no organisms in it that will initiate the composting process. Therefore it is the wrong thing to put down your toilet. QED

THE PERFECT COMPOST TOILET MEDIUM HH-5 from Good Gardeners International
you can buy it here