I do quite often sub all the flour. its a denser, chewier biscuit you get, but I like it.
I do sometimes roll/flour with wholemeal flour, but thats about it.
largely cos the kids like grinding up the oats
afaik sugar has no special magical powers, at least not til you eat it...you need it in bread, obviously, but not in biscuits.
I find, with oats, if you add chunks of sweet things (dried fruit, chocolate etc) you actually don't need the sugar sooooo much because oats taste quite nice alone
I am another one who will eat grapefruit raw though, so be warned...
franny, they are going to lose their horns when the biscuits rise, whatever they are made of.
I bake most days with the kids, so I can't be always doing white flour and sugar and so on.