I'm not sure why you would choose to use a tent rather than exit bag. I think it would be hard or impossible to get rid of all the oxygen and if it is possible, it would take more nitrogen to fill the bigger space so to me it just seems like a trickier method.
Even when using a bag, there's a lot of emphasis placed on flushing out the oxygen and even trying to empty the air from your lungs before trying to breathe the nitrogen, then you need the nitrogen to be continuously supplied for a number of minutes.