Yeah, I get this problem too. Not sure what the solution is. I'd like to know how to avoid it too. People might say plastic pants but I don't want to use those. I think the only time I haven't leaked after sitting down in a used nappy is when I waited for ages, perching uncomfortably on the edge of my chair, until it absorbed more deeply or did whatever it did during that time!
Would it worse for females - leaks while sitting? I can't get a bladder-full in an otherwise decent nappy, when sitting, because it can't seep up to the back and it can't travel far up the front. It's all concentrated on the middle part, where it is put under the pressure of sitting on it.
Well, if you're wearing a diaper, you shouldn't be holding it until your bladder if full. Thats your first mistake. You should go as often as possible, then your diaper only has to absorb small ammounts.