My advice would be, if you want to experience being in a messy nappy at night or waking up messy, then try putting oatmeal in your nappy, as you get changed for bed. It isn't harmful to the skin (as long as it isn't too hot to start with!) so you don't have to worry about how long you stay in it, and it doesn't smell at all. Plus you can make it whatever volume and consistency you want.
The only thing you'll miss is the experience of actually 'going' (although that doesn't matter as much if the idea is to mess in your sleep anyway, or if you want to know what it feels like to mess in bed, you can do that without sleeping the whole night in it). And of course you'll know it isn't the 'genuine thing'. But if I were you I'd consider it.